1996 Ram 1500 4x4 single cab short bed, 5.9 magnum rebuilt bottom end, eddy heads, mild cam, 1.6 summit rockers. New trans internals too from Monster transmission, 46re rebuild kit with shift kit and new TC from same company.

I get the truck up to 45 mph and I can keep accelerating just fine. But if I try to maintain 45 the truck just starts shuddering. Not in the steering wheel but the whole truck. I can give it more gas and it goes away, the same if I were to let off the gas completely.

This is an intermediate problem but it happens about 80% of the time. Any help would be awesome

 

Sounds like an engine miss. 45 mph is when such things are most apparent, as you are in O/D, and TCC is locked. Feels like the truck is trying to shake itself apart.

 

It doesn't feel "laggy" like a miss. And I don't hear any backfire?

 

If it's just one cylinder, or even two, you may or may not notice..... (I had two plug wires crossed, truck drove fine, but, when it hit O/D, holy smokes......)

 

Have you checked the u-joints yet? How about the differential? Pull the drive shaft, pull out n back in on the pinion yoke, is there any play?