I have some pretty substantial leakage from my differential. It is coming out from the pinion area I think. I am planning on changing it this weekend. How do I know if the bearings are still good. I plan to change the seal, gasket and fluid this weekend. I would like not to have to rebuild my rear end.

How can I tell if it really is just the seal? How do I know if the bearings are still good?

 

It's fairly common for oil to work its way through the
splines of the yoke and leak out around the nut-
sometimes you can clean that area real well with carb
cleaner then daub oil resistant silicone rubber in
there.
First though better check the tightness of the pinion
bearings-if the yoke moves up+down or in and out
any the bearings are too loose..
Also feel for any bearing roughness when you rotate
the yoke.

 

And check the pinion nut to see if it's loose. These back off often. If you have a bad bearing then changing the seal isn't going to do much for it.

 

If the pinion is secure and there is no play, could it be just the seal?

 

Yes.