Pretty rusty rear differential. Seeping oil that covers only the bottom 2/3rds of the back of it. No idea where the leak is, or whether it is just penetrating thin rust somewhere. Drips down on the pavement when sitting, not that much, but when driving is coated with oil in maybe 10 miles. Managed to get both plugs out, drained what appeared to be almost half a gallon. Refilled with 70w90 mobil 1.
Sure enough, seeping began again. I don't know how these are constructed. It appears there may be some kind of cover over the back, can't see any fasteners, possibly rusted off. Have no idea if the rear cover can be replaced.
Put 600 more miles on it but loooong way from home in a very rural area with limited parts, still leaking oil.
Runs pretty hot on a hot day, a bit hotter than the front differential. Can be touched and nowhere near smoking temps.
So any ideas as to what may be wrong and whether to try to drive it home and risk getting broken down on the road?

Pinion seal, rusted cover, or RTV failed where the pumpkin meets the housing. Check your breather. Honestly if it's already rusty I would just look for a used axle to swap in. Maybe more expensive but paying for convenience. You can sometimes find one for under 500.

Well the main concern here is trying to get 600 miles to home without a catastrophic failure, where I can work on it.
There is no oil on the front of the unit, only on the rear. Seems to be leaking from right behind the parking brake cable support. Have not been able to find a decent diagram to see where the breather is. The back cover seems to be welded on and it does not seem to be leaking from the front. Not sure how a pinion seal leak could escape through the back.
Anyway, getting home in it or towing it are the alternatives.

Pinholes in the rear cover are fairly common in rusty areas. It is not a bolt-on part, but you can get replacement (cut off and re-weld) rear covers.

Just keep it near 'full' for the drive home.

-Charlie

In your first post you drained approx 1/2 gallon from the diff so it doesn't sound like its leaking too bad. A 4wd w/o diff lock rear differential has 2.59 qt capacity which is .64 gallons. 2wd and 4wd with diff lock has 2.91 qt capacity which is .72 gallons. It sounds like you don't know how sudden or catastrophic the leak is so I'd keep a close eye on it on your way home. Also, the breather sits on top driver side of the differential.

Clicked on thanks and likes for all but for some reason my browser isn't accepting it. Will have to think about it...

Living in ontario canada we have pretty bad rust,
Almost the entire pumpkin on my rear diff is jb weld, had too many pinhole.

I cleaned it up really good with a wire wheel, wiped with acetone and jb welded the entire thing.

It's been a year now and no issues

Wow that is very encouraging. Thanks and a like. Do you happen to know how thick the metal on the pumpkin is?

Sorry, I don't know how thick it is, mine was pretty rusted and had some chunks falling off it. If I had to guess mine was probably a millimeter thick at some parts where I had pinholes and could see into the diff.

When wire wheeling some of the chunks that fell off were plugging other holes that weren't leaking so I used a whole tube of jb weld and smeared it over the whole pumpkin.

Honestly the easiest and cheapest fix I've done to my runner and has held up surprisingly well for the last year, plan to get a new pumpkin welded on but I don't have the skill or the money.