Hey all, ive got a 94 Samurai with (as near as I can tell) a sidekick rear third. This third was in the sami when I bought it, and recently started making a kind of clunking noise and vibe on coast and at speeds above 35 mph, regardless of whether im in 3rd or 4th gear. No noise or vibe is present in 1st and 2nd, even when wrapping it out against the rev limiter. Since this isnt RPM affected, and due to the noise involved, im pretty certain its a bad gear setup on the diff, and im looking at getting it rebuilt, or doing it myself. 

I was just wondering if anyone had experienced a similar problem or had any other ideas?

PS: not a bad U-joint, thats the first thing I checked.

Not doubting you but did you check all 6 u-joints? Also how did you check them ? I usually run it and then feel them. If it's going bad, it will be hot. They are sometimes not that noticeable visually. The shaft between the tranny and the t-case is hard to check any other way.