I have an 84 6.9 4x2 with a dana 61 read end. When I shift I get a thunk like a bad u-joint, but all the u-joints are very solid with zero play. Also, under load (primarily in 1st and 2nd) I get a clicking noise. I laid under the truck with a friend rocking 1st to reverse and the thunk appears to be in the diff. The yoke has nominal rotational play and no in/out or up/down play and the diff has oil. Any idea's?? Daily driver that tows a 3k lbs trailer.

 

could be play in the spider gears or the cross pin. pop the cover off and take a peek.

 

I got under it to check the yoke nut and the rear u-joint was stiff and chunky. It wasn't lube-able so I replaced it and everything seems OK. The bearings were really dry, and it had no zerk. I like the $15 fixes.

Thanks for the input.