Just a guess, but something is probably warped or out of round. The first step is to isolate each differential to find which one is doing it. Pick the front up, hold both tires from rotating and roll it on the backs, still do it??, then its not the front. Do the same thing with the back, still do it?? then its not the back and may be the center (try to isolate the front and rear from being the problem if at all possible first). In all likelyhood, it is just a screw that holds the ring gear onto the differential carrier, but finding the diff that is the culprit is going to be the interesting part. Heck, its probably time for new diff fluid anyway, so pull all three of them apart and have a look.