How's your steering? If you feel it in your steering wheel, I'd look up front. If it's the chassis that's shaking, check the rear. I have a bad Idler Arm on mine. It makes the steering a little loose and i get a shake at around 75 MPH. I don't know if that helps; but, I hope so.

 

Today i took the back drive shaft out and checked the u-joint. They where in great condition. No looseness at all.

I also lifted the back end up and supported there rear axel, so the back wheels where in the air.
I did not get any vibrations at the spec speed and RPM.
Even checked the drive shaft for any dents or bends. It was 100% straight.

So now what could it be?

Thanks

 

mud buildup on the inside of your front wheels, trannymount, transfercase mount.... just thoughts