my rear diff has always leaked a little bit. it builds up oil at the bottom of it not enough to run the housing dry just makes the bottom of the cover oily. my question is what would be best to seal it up? thanks

 

Is it coming from the cover gasket or pinion seal....

 

i belive cover seal

 

You will have to remove the cover to fix it, so do a lube change. You need 75w140 synthetic gear lube, and if it's a LSD, a 4 oz bottle of the Ford additive. When you are at the Ford dealer, get a tube of their gasket making material that's made just for those rear ends. Clean the cover and mating surface thoroughly and make sure the cover isn't warped before resealing it.