1996 Chevy Astro changing rear seal

The tail shaft seal? Remove driveshaft, pry old seal out, use aseal installer or flat piece of wood to tap new one into the tail shaft. Use emery or scotch-brite to polish the driveshaft yoke where the seal rides to assure a good sealing surface. The only other thing is to try moving the driveshaft front yoke up and down when in thetrans to see if it has excessive play due to worn tailshaft bushing. It should move a little but not very much. If sloppy, you should have the bushing replaced