[lincpimp]

Ok, moving the grub screw will not help.

The lst pinion needs to have the couple tight against the inner bearing race of the outside bearing. If not the pinion will "walk" into the diff case and cause mesh issues.

I have the original rcm steel couplers like you pictured, I will get a shot of them at work today and you will see what needs to be done. The center hole needs to be drilled to 8mm all the way thru. that way you can slide the couple down and have it touch the inner race and the pinion will stay centered between the bearings.