The radiator in my '99 has severe corrosion. It looked like Yoohoo when draining it. Also noticed the plastic tank is starting to crack at the top. I found a solid aluminum radiator for the truck that isn't much more than one with plastic tanks. I'm also getting a failsafe thermostat. Only issue with the radiator is a note that fan blades may need to be shaved down for it to fit. My brother suggested I look into a slim replacement electric fan. Found these possible choices (Amazon links below). Need advice on which fan to order. New radiator: Core size: 21.00"W x 23.50"H x 1.65"T / Overall size: 26"W x 25 1/3" H

• https://www.amazon.com/dp/B079NJ5X9W/ref=cm_sw_r_cp_apa_fabc_CYACN0XKXKHTG4CQEJCZ?_enco ding=UTF8&psc=1
• https://www.amazon.com/dp/B014KK7LDY/ref=cm_sw_r_cp_apa_fabc_Q3N7Y4YH2ZW4203XZJSA?psc=1
• https://www.amazon.com/dp/B07F1N2WPC/ref=cm_sw_r_cp_apa_fabc_MY9S113ZHBNDEY6RM2DN?psc=1 (includes thermostat relay wiring kit)

 

You've got a 4.0L OHV; my 2001 has a 4.0L SOHC; may be the same factory radiator?
Last year that junk PlasticTanked Radiator split along the right hot side tank & drain;
replaced it with a 4CoreAllAluminum; best thing I ever done for this truck.
Also replaced RubberHoses with StainlessFlex. Was a fairly easy drop in replacement.
Thought about an EFan, but opted to keep original engine driven fan+clutch arrangement; did not have a fan clearance issue.
Did modify the fan shroud; made it a split upper\lower half for easier removel\install.
Had to carefully measure core & Top+Bottom channel dimensions to get the right radiator.
Sounds like your new radiator dimensions don't match your old one?
Considering your "yoohoo" colored old coolant,
I'd first do a full flush of your coolant system & replace any other old components,
so to not contaminate\clog the passages in a new radiator.

More info at
https://2001fordexplorersporttrac.fa...t_Radiator+Cap
https://vignette.wikia.nocookie.net/....2173_4of4.GIF

 

Haven't bought a new radiator yet. Friend of my brother knows someone who bought an aluminum one for his '98 Ranger that had a tree fall on it before he installed it. So he's gonna see what I can get that one for since their Ranger was smashed.

I'll see about the stainless hoses. And flushing the system is first on my list, already got a bottle of Prestone flush. I was considering an e-fan cause I can get a slim one which would allow a slightly wider radiator.

 

Your radiator is made of aluminum. Your block is cast iron, which is the source of your "YooHoo" colored coolant. Your coolant was likely never changed and lost its anti corrosion ability. Pour in some Prestone flush and drive it around for awhile, you will have to do a complete flush with distilled water. ChrisFix has a good tutorial on it. Good time to change your thermostat and housing too. My coolant got rusty too, did a complete flush and its been fine ever since, you likely won't need a new radiator. Since I used distilled water to flush it I had to use the concentrate to get it near 50/50 since the block still held most of the water.

 

I'm replacing the radiator because the plastic tank has small cracks on the driver's side. I'd rather replace it now while the truck is down than have it bust on me while out of town working. Definitely plan to replace the thermostat, I like the failsafe "hold open" over regular thermostats.
Stock radiator info:

• Core Height: 18 3/8 in
• Core Thickness: 1 in
• Core Width: 18 in
• Fan Included: No
• Inlet Location: Top Right
• Inlet Size: 1 7/16 in
• Outlet Location: Bottom Left
• Outlet Size(s): 1 1/2 in
• Row Quantity: 1
• Transmission Cooler: No
• Tube Size: 1 in

Since the stock radiator doesn't have a built in tranny cooler, would an upgrade/new tyranny cooler help it run better? I'm willing to spend money now to extend the life and improve performance.

 

Your stock radiator is smaller than mine, so I now see why you're having fan clearance issues.
The TransATF~CoolantHeatExchanger really is used to warm up the ATF quicker during the first few minutes after a cold start.
Considering the coolant temp typically runs 180~200f & the internal Thermostat in the 5r55e doesn't allow external ATF flow until the tranny reaches 150f,
if you currently do not have an external ATF loop running thru the radiator, I would not add that; it will not give you extra tranny "cooling".
You can add an external TrannyCooler (smaller radiator behind the grille) if you dont already have one; or a 2ndParallel or Larger one.

 

A note on replacing the two radiator RubberHoses with StainlessFlex;
I needed new hoses anyway & did this to further increase the effective cooling surface area of the radiator.
I bought a kit off Ebay with 48" of 1.5"ID SSFlex & some fancy chrome covered end clamps.
The chrome end clamps were junk, but the flex itself was good, easy to cut & bend as needed.
Used 4" section of standard 1.5"Coolant hose & doubled up the worm clamps to make those end connections.
That flex does get hot; so use a short split piece of rubber hose & zip ties where needed as a heat shield,
so not to melt any nearby plastic components.