I have a 1988 GMC C1500 2wd that the transmission extension seal keeps leaking. ( where driveshaft goes into transmission) i have replaced this seal 3 times with in 2 years. do i need a transmission repair shop or keep replacing the seal?

 

check your driveline angles
check for noise/play/wear/vibration in the driveline (eg U-joints, pinion bearings, transmission mounts, etc.)
Make sure the sealing surface on the shaft is smooth.

 

To add to MMJ’s advice:

Check the output shaft for play top to bottom and side to side. Less than 1/8” play is normal - more than that indicates the shaft bearing is worn.

Also verify you’re getting the correct seal for the transmission. In the glove box should be the RPO sticker - look for the code that starts with M (an 88 may even have the descriptions printed on the sticker) to verify (assuming it’s the factory trans).

Are you lubricating the driveshaft yoke with ATF before installing it? If not, the seal may be getting torn or damaged during installation.

Have you noticed any vibrations during acceleration? If so, the u-joints may need replacement.