...checked if it was engine related by shifting down into 2nd gear and bringing up engine RPM to same RPM as vibration in 3rd gear(starts at 55 MPH 2800 RPM)...no vibration....:axe:
That tell me vibration related to driveshaft speed, not engine speed. Have you tried driving 2nd gear till RPM reach 4060 (2800x1.45), I guess it will vibrate @ 4060 RPM as well.
If so, the driveshaft is the problem:
1) the shop did not do good job balance driveshaft. It might show "balanced" at the Balance Machine at lower(higher) speed but it does vibration at certain speed (in this case 2800 @ driveshaft)
2) The driveshaft is too short. The faster you drive the longer distance between end of tailshaft & rear end will be. Therefore the less connect between the tailshaft and the yoke.
