You say you "lengthened" the shaft. Please explain this. I dearly hope someone did NOT actually lengthen the tube, as opposed to starting over with a second shaft
Generally, play in the trans slip bushing, bent/ worn slip yoke, improperly installed U joints (pins bent in the caps), worn U joints, bent yokes on the shaft, bent tube, out of balance tube, and bent yoke on the differential. Play / worn pinion bearings
How did you check shaft length? You must do this with car weight on wheels, on a flat surface, NOT jacked up with axle weight "hanging."
Shaft could either be too short or too long. Too long-- yoke can bottom out on trans shaft Too short, yoke can be extended off end of splines so far that it doesn't run true
Tell us about how you checked pinion angle? Basically what you want is for the trans output shaft and pinion shaft to be parallel when under acceleration.
