379 peterbilt problems

I have the same problem with reg air leaf and 630,000 miles just changed u joints, clutch, input shaft and carrier bearing.I raised both rears off the ground to look at the wheels spinning rear axle shakes a lot more when only rear is off ground when i raise both rears off ground it smooths out a lil bit
I had this problem on a 94 pete and never found out what it was after changing many parts then trading it in....now i have a 2003 with the same problem

 

If you have the Peterbilt air suspension check the u-bolts in the front of the springs just behind the spring bushings those are notorious for breaking when those break it will feel like you have no suspension and it will rattle and bounce

 

If you have the Peterbilt air leaf suspension check the 4-in u-bolts right behind the spring bushings they are notorious for breaking and when they do they will produce a bounce and shake and it will ride like there is no suspension