Rear End Clunk

I have a 1991 Chevy K1500 Automatic 4x4

There is a clunk in the rear end, it only happens when I take off from a stop, or sometimes when I slow down a bit and give it more gas. It doesn't do it at all when i'm just driving, it happens when I slow down, then start off again.

It think it's something in my differential, on the cover plate there is a long gash mark from the ring gear (8.5" diameter) and a bunch of small nicks on the side; I'm not to sure what it is, i've heard the it could be the yoke bearing.

There is only gear lube leaking from where the yoke goes into the differential, I know that I need a new seal there. But there is no lube coming out from the differential cover plate. There also isn't any large gashes or scraches or nicks on the gears in the differential, except on the edge of the part where the pinion shaft goes trough, but I think it's been there because there wasn't any metal pieces in the differential.

Any help is greatly appreatiated, Thank You