This is an interesting post because my newly resurrected truck (55 first series) may have the same issue. I was thinking it was in the transmission until I read this post. I have not removed the driveshaft yet so I cant check for in an out play of the yoke but rocking the truck back and forth with my hand on the yoke, I can feel a rather unsmooth mesh of the pinion and ring gear. Its probably the only part of the truck I haven't replaced yet. I will post pictures once I get the cover off. Jerry - I may need some pointers:-).
Joe
