[BADRACR1]

I ran across a 1976 model with 86k miles. All stock, no mods.
Paint is good, very minor bump to the nose (scuffs and some hairline spiderwebbing but no breaks or tears). Interior is really nice, needs a good cleaning and detailing. Only real bad spot is where the left speaker is. Perforated area has a chunk missing.
AC is cool but not cold. Windows work. Doors shut well.
Cons are small exhaust leak on passenger side, tach doesn't work, and the issue I am curious about: runs and drives out very well, but when first taking off there is a shudder feeling from the rear end for a few seconds, and when backing up it will make two or three popping sounds. Looks to be an axle seal leaking somewhat on the right side coming out of the differential. Not dripping, but one can tell it has a leak. Fluid level is good in the chunk. No noises or vibrations when driving, only when first pulling off from a dead stop.
Local shop says likely spider gears and he can repair it for around $400/500. States it is a common issue. They have an '80 in now with the same issue. My concern is it may be worse, and I know that gets expensive.
Anyone have any experience with this?