Travis I just had to set of Nitto Mud Grapplers that were on my truck when I bought it and the truck always had a shake at 45mph anything less and you wouldnt notice it and if you went over 45 more like 50mph it went away. Tried getting the tires rotated and felt the vibration in the rear. The tires were cupped pretty bad from non rotatation and possible bad shocks(which for me I went to a dual shock setup recently). So needless to say it was my tires. So you might have the rotated from to back and see if your vibration moves or have your tires checked for balance. Bought a new set of BFGoodrich Mud Terrain TA and the problem went away.