My 92 Astro leaks around the rear seal when on an incline also ( am on my orginial transmission). I believe that this is due to the seal rubber getting old but I haven't changed mine yet because the trans fluid level is still ok.
I also think that this can be due to the trans fluid level being a bit too high which would cause the excess to leak out of the rear seal when on an incline. Double check your trans fluid level - if you see the level going down then replaceing the rear seal would be cheap insurance.
If you drop the driveshaft, reference mark prior to disassembley since it was factory system balanced.
Noise or vibration at high speed are indications of a balance problem.
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles