problem solved: bad U-joints

How did you check 'em? I learned a LONG time ago you must remove the shaft to check joints. It's the only way you can feel movement in them.

Another thing to watch in U joints is that they don't get "out of time" You should be able to sight down the shaft like a rifle barrel and the joint caps should be perfectly in line, rotationally.

Sometimes these stupid "rubber connected" shafts (tube in a tube) slip, and SOMETIMES the machine shops are sloppy

Another thing that can cause stuff like this, other than pinion angle and bad pinion bearings or transmission bushings is if the shaft is either in too far or out too far on the slip yoke.