I'll start with everything I noticed from the beginning. I went and test drove an automatic 2002 Ranger 4.0L 2x4 in August this year, it ran good (not great). Started right up, wasn't necessarily the fastest thing I'd ever driven but it could get up to speed reasonably quick. There was a slight hesitation on idle, but other than that, gears shifted smoothly and once you were driving that hesitation was gone. I live by myself and had to make a trip due to a family emergency, and I did good this year so I decided I would finally buy my first vehicle. It went from North Carolina to Delaware and back with no hiccups other than my brother spilling coffee on the unstained tan center console.

Fast forward 2 weeks. Had an old friend I used to work with flying in for a vacation so I went to pick him up at the airport. On the way up, about 15 minutes into the drive, my truck stalled at a red light. It started right back up and didn't stall for the rest of that drive (approx. 2 hours). After that, stalling became a regular thing. Sometimes you'd go an hour without stalling, sometimes it was five minutes. I eventually took it to a shop wondering if maybe it was running lean or rich because that was a big issue with my brother's old BMW. They ran through a bunch of tests and got back to me saying my fuel pressure was perfect and there were no vacuum leaks. They test drove it and ran it for 45 minutes and it didn't stall once. Drove it back home with no problems. The next morning I went to start the truck, and it cranked but didn't start. I called up my brother and he told me to give it a little bit of throttle when trying to start it, a little over 1,000rpms but under 2,000. I did exactly that and it fired right up. He told me to clean the IAC valve and throttle body. I called the same shop and the guy said he'd do it for free. When I left I didn't start the car since the tech drove it out to the parking lot and left it running for me. I drove back and it definitely seemed a little healthier, but I wasn't too convinced.

I kept driving the truck (I drive no more than 30 miles a day, I pretty much only drive between work, the gym, and the grocery store which are all close by) and it seemed like it was doing fine, but still not doing great. Fast forward to last month, right after Thanksgiving. After spending time in Virginia, I made the three and a half hour drive (that's without the added holiday traffic) back home. The speed limit for a good 75% of that drive is 65-70mph. I drove for about an hour and a half, using cruise control once I got on the highway, and pulled into a rest stop for a drink and the men's room. Merging back onto the highway, the right lane was moving around 10 under the limit, and I found room to merge in the middle lane. I gave her some throttle and while trying to accelerate from 60 to 70, my truck lost acceleration. I was luckily able to pull to the right shoulder with ease. I had no idea what to do and had no AAA. I tried to see if it was a one time thing by accelerating while I was still on the shoulder and couldn't get out of first gear, engine was roaring trying to maintain 30mph. While doing this I also heard a sort of metal grinding noise coming from below me. A tow truck driver saw me on the shoulder and pulled over to see if I was alright. I explained the situation including why I took it to the shop. He checked around a little bit and then checked my transmission fluid. It was way above the cross hatch on the dipstick. He told me he could tow me wherever I was going thinking I was local, but after I told him where I lived and he told me how much it would cost, I asked him if I could just limp mode it on back roads the rest of the way and he said it was worth a shot. I gave him 10 bucks as a thanks for checking on me and he gave me his business card in case the truck didn't make it. Four and a half exhausting hours later I had finally pulled into my driveway.

I knew I had to get my vehicle fixed but I just kept putting it off. It was finally this past week that I started calling local shops after I realized it was almost time to go back and see family for Christmas. Everyone's been booked without openings. Two days ago I called the only two mobile mechanics in my area that I could find online. Last night one of them gave me a call back and I told him all of the above. He said he could come out today and see if the overfilled transmission fluid was the culprit all along. He came out, he drained 5 quarts of trans fluid and put back 4, we went on a test drive, the grinding problem persists, BUT we reached 55mph. He was a little confused by the grinding since he said he found zero metal shavings in my transmission pan. He told me it might be a valve body, worse case scenario it might need an entirely new transmission.

I'm writing this pretty late, it's 10:00 at night, so tomorrow I will have 2 days left till Christmas Eve. I really am hoping not to have to find a new transmission. I'm semi-confident I'm able to drive it 55 through back roads all the way home, but if anything I said in this post rings a bell, please please please share some advice. I don't have experience working on cars other than changing oil but if anybody can guide me towards solutions I can definitely GET some experience following posts on this forum and posts on YouTube.

EDIT: Forgot to mention; When the grinding noise started, it started as soon as I tried to accelerate that one time on the highway (see 3rd paragraph) and then when I was stuck in first gear driving 30, any time I gave more throttle to try and exceed 30, it would grind. Now, still grinding, I was able to reach 55mph without grinding, but grinding occurred when trying to exceed 55. The truck was successfully able to shift into at least third gear, but I wasn't able to hear or feel it shift into fourth as it was windy tonight.

 

How many miles are on your 2002 Ranger? Also include a general description of the overall state of maintenance / condition of the vehicle.

I believe your symptoms fit those of a worn / loose TPS (Throttle Positioning Sensor). They're quite affordable. I'd stick with the Motorcraft brand.
There are also "cable shortening" shortcuts (a small zip-tie behind the end bead at the TPS can effectively shorten the cable IF IT IS STRETCHED.

You want to clean the MAF sensor with MAF spray. Many electronics cleaners leave a coating of lube / anti corrosion stuff. Get MAF cleaner.

Noises are a bear to describe in words.
Was any element of the noise speed related?
Could you place the vehicle in neutral while it's moving and grinding?
Do you have any idea of the direction of the source of this grinding noise?

While idling, look at the accessory belt and rev the engine to see if an idler or tensioner is grinding.
You might see the belt hop unsteadily or the tensioner wobble erratically.

Check the power steering fluid level...again, it can be a true grinding or what some might call a P/S grumble.
Check for real loose splash shields...or bright/polished pully cheeks.

Also, cats can clog, and any resulting exhaust leak can resemble darn near anything, EVEN A GRINDING NOISE (believe it or not).

With the engine off (obviously) grab and wiggle the alternator and any other accessory to check for solid mounting.
This goes double for the fan and its viscous coupling.

I doubt your transmission is toast, but it would be nice to know the history of the overfill.
Where'd you source this truck?

Two touchy points:
1) You said, " and I did good this year so I decided I would finally buy my first vehicle". How have you been getting around?
2) Shorter posts get more responses. Edit out fluff (Ex: 1st sentence of paragraph #4).

 

Nice.