Ok,
A. Is it difficult?
No. Transmission doesn't have to come out. Get it up in the air, pull the pan. And for all of you reading this, even if the rear main isn't leaking, you should pull the pan. The picture below is a customer's M38A1 that was cruise night quality and ran fine........... until the engine locked up from spinning the #4 rod bearing. And when I got the pan off...........
I can't tell you how common this is. The pan below is from an M2A1 halftrack I'm working on:
But, in answer to your question "is it difficult?", no.
Get it in the air, get the pan off, clean it out. Check the rail for flatness. They're all warped from people overtightening them. Get your metalworking hammer out and make sure the flange rail is perfectly flat.
With that done, you'll have to remove the rear main cap. This will give you a chance to see what the bearings look like. With the cap off, you can carefully spin the upper half of the seal out of the hole and remove it. the lower half is obviously in the rear main cap.
Inspect the surface of the crank where the seal rides. They can wear out and if they are, nothing is going to seal.
Clean the heck out of everything. Brake cleaner works wonders. Put a little bit of grease on the new seal and a little bit of RTV on the backside (where it is pressed against the block). The idea is that when it is installed, you want the RTV to help keep the seal from trying to spin or oil get past it and the grease is so it doesn't get hurt spinning against a dry crankshaft. Same goes for the bottom half.
Before you reinstall the cap, there are two rubber dowels you have to install. They slide up into the block. Sometimes, you can compress them enough to work, I've had a couple I had to trim a bit. Put RTV on them too, but don't overdo it.
With that all done, install the cap, torque to specs.
Install the oil pan with the new gasket and glue both sides of the gasket. Torque it to spec (you'll need an inch-pound torque wrench) and let it sit and cure for 24 hours before adding oil to the engine.
If you care about how your work looks, take a straight razor and trim off the excess sealer that pushed out from in between the pan and engine and touch up the engine paint. No point in doing shoddy work.
Jeeps have a rep for rear main seals that leak and I've done 6 and none of them leak.
Pictures of the process:
Get it where you can work on it.
Pan obviously leaking.
Pan off.
Cleaned pan and rail straightened:
Rear main cap before removal. Notice a lock washer that's lost part of it's body? This is why you rebuild stuff that's 75+ years old and you don't use used hardware. If this bolt had lost tension, it'd be a bad (expensive day).
Rear main cap and new seal and inspecting the crank seal surface:
Pan repainted and installed:
Gasket sealer trimmed off and paint touched up (not finished in the last picture):
B. Which one do I buy (rear main seal)?
The ONLY rear main seal that is currently being sold that is neoprene (i.e. not a rope seal) and that works is the one from Midwest Military. They tooled up and made it. I've tried Fel-Pro, National, no-name Chinese repros and nothing is correct. If you go onto G503.com and look in the Jeep section, there are multiple stickied threads about all the incorrect rear main seals being sold. They're either too tight and the engine won't run (or they'll burn up the crank/seal!) or they leak.
This is it:
https://store.midwestmilitary.com/product-p/7372560.htm
