9" pinion oil seal

Can it be replaced DIY? Or should I take the 3rd member somewhere to have the seal changed?

HOw can I tell if its bad? The truck sat for a unknown amount of time before I got it so I am afraid of it being dry rotted.

My 69 will drip if I leave it sitting for a while but when driven alot it don't leak any been that way for 3 or4 years now I just check it when I service it if it not leaking bad i would drive it alittle and keep a check on it it may stop after driveing it alittle.just my 2 cents

If your third member has a solid pinion spacer sleeve instead of a crush sleeve swapping out pinion seals is damn easy. Since all the clearances should stay the same if reusing a solid spacer when reinstalling/tightening the yoke after the new seal is in, there shouldn't be any problems.

But there could be a problem if it has a crush sleeve since a used crush sleeve might not offer enough compression resistance when reinstalling/tightening the yoke making the pinion bearings too tight. If this happens sometimes you can add a thin spacer/shim to the crush sleeve adding back some compression resistance.