drivetrain shudder? driveshaft length?

I don't think being too long would cause the shudder. Could very well be pinion angle putting too much angle on one (or both) u-joints. Be sure its in the drive line. Run the engine at various RPMs while in neutral and see if you get any vibration there. If not look at the drive line. Here is something else to think about. Depending on whether the tranny has has a lot of abuse, the sprag could be trying to spin in the case. Not a frequent occurance but I have seen it happen.

Larry