Don't Know Where To Look

[ramman54]

2001 ram 1500 quad can, 4x4 5.9, auto, stock suspension, 285-75-16 AT tires on stock steel wheels.
About 3 years ago it started to develop a vibration at 45 mpg.
Bought new tires and spin balanced. Installer said he thought it was a bent rim but couldn't remember for sure. Wanted $15 per tire to check on spin balancer again. Same thing but sometimes gets worse as speed increased, sometimes hardly noticeable. Swapped one by one with the same size AT spare and also rotated the tires. No difference. No slack noticed at u joints. New calipers too. Where to look now? TIA.

 

[Gr8bawana]

Perhaps an unbalanced or bent driveline.

 

[dudeman2009]

Take a look at the driveline. It should be straight with no dents or bends. When you checked for slack was the transfer case in neutral on level ground? Did you just try to turn the driveshaft or did you also try to push it up and down, side to side? Look at the trans mount, is the rubber part of the mount in good condition or is it rotting out? Check out the rear axle, does it look like work has ever been done to the spring seats or axle tubes? I have a driveshaft vibration due to a couple factors now, but it started a couple years ago after I hit some road debris and dented my driveshaft. Now I feel a vibration right around 40-45mph but only once in awhile, certain accelerator conditions cause it to be more pronounced.