Source of differential leak and repair advice
 

I have a 1980 and have noticed a small oil puddle under the right side of the differential. I have checked for leaks under both axles and the pinion seal and cannot see any oil. I cannot see the pinion seal but put my fingers around the front seal area and do not feel any oil. The rear spring on the right side is a bit oily and the right side exhaust pipe had some oil.

I thought the fill plug maybe leaking. I raised the car up so it was level, removed the plug and some fluid ran out, so I figured it was overfilled. I waited for all the fluid to drain, then put some thread sealer on the plug.
I used brake cleaner to remove the gear oil from the rear spring and dried everything off.
I drove the car for about a half hour around town (30 to 40 mph) and parked it overnight and no leaks.
Today I drove the car for about a half hour more steady at 45 to 55 mph and parked it.
Looked underneath and there is a small puddle in the same area. There is no oil around the fill plug, but the rear spring is oily again and the right exhaust pipe under the differential is a bit oily.
What is the source of the leak?
Can it be the gasket around the rear cover?
It is beyond my ability to pull the rear end so I would need to take it to a shop.
The car has 37000 miles.
Should I have a shop replace the rear cover gasket, axle seals and the pinion seal or have the differential rebuilt? The differential is quiet and smooth.

I would have them just drop it and re seal it. Inspect the yokes for wear while they are out. .030" is normal.

Does it sound like the cover gasket is leaking?

My point is you have to drop it to change the gasket. If you drop it to fix anything, I would not rebuild it, but might as well re seal it. It's out of the car anyhow. Then, any leak , no matter where is repaired.
If you reseal the sides, you have to pull the side yokes, so check them.
Call it a Diffy tune up.

It might be easier to pinpoint the leak if you clean the differential, dry it off and dust it with baby powder.
And I agree with Bird- at 37k I bet the inside of that thing looks like new. Seal it up and drive.

[QUOTE=TimAT;1595159664]It might be easier to pinpoint the leak if you clean the differential, dry it off and dust it with baby powder.
And I agree with Bird- at 37k I bet the inside of that thing looks like new. Seal it up and drive.[/QUOTE

If the differential vent is plugged up, can that cause pressure to build and a leak?

Since it didn't leak on a lower speed drive, but leaked on a higher speed drive when it was a bit warmer can the differential vent be the cause?

The vent would have to be 100% plugged solid IMHO. And the case is only about 1/2 full of oil when full. The oil level would have to be up over the seal areas.