Hey everyone. This is my first post on these forums and need some... enlightenment.

I have a 97 xlt 4x4 with over 280k miles. My gf and I were getting off the highway 60 miles from home and as we came to a stop at the end of the ramp, there was considerable grinding coming from what I thought at the time was the differential. I drove it to the restaurant, and over 3 mountains to her Grandparent's house and it never did it again.

We drove home today from VA to TN and didnt have a problem. I did stop at Walmart to check the trans fluid, and I was a little low so I added a quart. I also bought differential gear oil, but didn't add any. Fast forward to later in the day when we drove to the pharmacy to get her meds. She's got pneumonia. Anyway, as we are heading home, I'm pressing the accelerator and it's just revving up no acceleration. I was able to pull over, I shifted into park, then into drive, and it would shift from 1st, to 2nd, and then rev again with no acceIeration. I pulled over again, shifted into park, then into 1st, and it would go just fine, shifted manually into 2nd, then into drive, and it would do it again, just rev and no acceleration, but this time, when I put the shifter into park, it wouldn't shift the trans into park, or any other gear. (When I shifted into park, it would grind, then stop, but not actually be in park. We pushed it to a safe area and I called a tow. The tow was about 10 minutes out and I had an idea. I put the truck into 4 high, shifted into drive and low and behold it drove. I took it out of 4wd, put it into D and nothing. It didn't move, it just revved up. So I put it into 4wd again, and it moved, shifted, and got us home

Here's the kicker. My front hubs DO NOT engage at all. My 4wd hasn't worked for months.
The 4x4 solenoid works.
the front drive shaft spins.
The front axles spin.
But the hubs won't engage. I just haven't bought manual hubs yet.

Why does my truck not move or shift out of what is essentially neutral, unless I have 4wd selected?
Is the transmission bad, or is it the rear differential?

Thank you.

 

Were I in your shoes I'd check the operation of the transfer case. Check its fluid level and condition.
You say it shifts OK when in 4WD? Then it's likely not the transmission.

Can you lift the entire vehicle on level ground and put it into 2WD D and listen to the rear end?
Have someone put it into 4WD while you listen at the transfer case.

 

If it drives in 4 wheel drive but not in 2wheel drive, your front hubs are likely working and something from the rear of the transfer case to the rear end has come undone. Of course you might have broken an axle or something in the spider gears, but rear ends are quite robust. Put it in park and jack up one of the rear wheels. Try to turn the jacked up wheel. It should not turn because park should keep the driveshaft still and the ground should keep the other wheel still. If you can turn the wheel see if the driveshaft is moving. If it is standing still the problem must be in the rear end.