Originally Posted by lookingaround
im thinking it started from the old drive shaft being to long. maybe put a stress crack in it. then when i put the shorter shaft in it and took it for a spin it done that? thats the only thing i can think of off the top of my head.
The critical speed of a drive shaft is reduced as the length is increased. You have the benefit of looking at all the variables in your garage and hopefully doing it yourself, so if you had a drive shaft vibrate at speed due to exceeding it's critical speed (RPM) you likely are correct on the cause. Some guys try to weld up there own shafts and do not have a way to jig the ends in perfect phase which causes the drive shaft to whip when it is spun. The other possible cause can also be using excessive pinion angle. Remember that the U-joints angles on each end need to cancel each other out by being within .5 degree of each other or less at load. Example, having 1.5 degrees between the trans / engine angle and the drive shaft must be equal to the pinion angle at the other end at ride height and at load + - 1/2 degree. The acceptable resultant angles could be 1 deg to 2 degrees going down the track on the pinion end for the 1.5 degrees on the trans end of the shaft. I am sorry if I made this sound complicated, but it is easy to over simplify, I hope I did not make it hard to follow so you need to read it several times to try to follow it but it is not an opinion, and is critical to getting all your engine power to the ground and not into mechanical bind.
Food for thought