could be a number of things as you know, but seeing you have some heat at the shaft front u joint, any binding/stiffness at the u joint-yoke when you had it out?...is it possible the driveshaft is too long/short?, how much fore to aft engagement of the yoke into/out of the tranny tail shaft/spline?, 3/4" is about ideal...how much spline of the yoke is in the tailshaft?