I need some advise

I agree on the pinion angle. Also, how long is the new driveshaft? I am guessing abnormally long, and a long shaft will have more of a tendancy to "whip", especially if there are any loose u joints, worn slip yoke bushing in the transmission (any play in the slip yoke?) Or incorrect u joint straps at the differential yoke (large and small straps look very similar and can be mixed up).