904 rear spline/yoke

Anyone know if a problem here..?? have persistent drive shaft vibration on hi mile 67 dart GT convert; found yoke a little loose on trans spline, put in another used one, it got a little better..for a while (10k miles) . These were made various lengths by MOPAR --? something going on. The long ones actually have a shorter spline inside than the short ones .
Drive shaft rebuilt twice,all new rear end bearings , just rebuilt , thought it might be pinion bearings, a little loose too.
Problem getting worse, rumble at 40 mph when balancing torque to zero w gas, ok under throttle or coasting down hill..OK,but not perfect .
May be chasing something someone else knows all about. Trans rebuilt by TA , first time wrong, but they made good on error. Probably same output spline put back in.

Any sources for new ones, (front yokes) that are wear resistant? comments sought... How to get tighter fit, if fit is the problem..