help!!
 

I have a stock drivetrain 99 cummins 24 valve 4x4. Has leveling kit with 35-12.50-18. At about 15-35 MPH it starts shaking and feels like the tires are spinning. The harder the throttle the worse it is. Truck has 213k miles engine was supposedly rebuilt 20-30 k ago.. I've checked everything I thought it could be. Its feels like its coming from the rear end. We jacked it up and put it in gear and it didn't make the noise. What else could it be??

Well check you U-Joints and carrier bearing, and look at the yokes, jacking the rearend up and putting in gear wont get you much because its proably happening underload. You can also just do a slight brake boost and see how things go from their.. DO you have a Manual or auto...

I Have an automatic. I had it narrowed to the rear end but didn't know where to start all the u joints look in good shape

You need to go yank on them push them up down and side to side and look for any play. Carrier bearing do the same if the rubber bushing moves thats not an issue. If that all looks good drain your diff and look at it.. If thats where you think the problem is.

out of balance tires maybe?

I'm gonna check the rear end this weekend and x that out if it ain't there..and I figured if it was the tires it would do it all the time

out of balance tires usually vibrate at certain speeds...

Ill run by the tire shop and check that real fast