Truck Repair: vibration transmission, slip joints, vibration transmission

Question
HI I have a 2001 peterbilt N14 460 hp with a meritor 10 speed smart torque 2 trans. I had a new clutch,throw out bearing ,clutch brake and 3 u joints installed about a month ago I just finished a trip dropped the trl. and started to bob tail home when I put it in 8th gear and stepped on it I felt a vibration and like a bit of a growling noise I let off and tried it again and it didn't really do it then I kept trying slowing down and go through the gears  if you let the rpm drop to around 1200 and stepped on it, it would start the growling noise and vibration as soon as you felt the tubo kick in then let off it would stop any help would be great thanks Jeff

Answer
I would first check your leveling valve in the suspension look at your air bags and make sure they are sitting about right not all the way up or all the way down. After that would check all your u- joints and carrier bearing for any up and down movement and also check your slip joints in the driveshaft make sure they are still slipping and not frozen up. I would doubt its in the transmission as long as your fluid is normal seldom do the bearings in the trans go bad. more then likely its one of the above just transferring the vibration up to the transmission.