i have a similar issue with my '11 overland. tires have been balanced and it's still there and the dealer claims the bearings check out okay. i feel a slight thump, thump, thump, at lower speeds and then a noticeable vibration at speeds 50+mph and really bad at hwy speeds (70+).
What wheel/tire pkg do you have?
I'm also considering picking up a set of 18" brand new Take offs just to see if the ride changes...it may seem crazy, but if purchased right, I could always sell later if i decide to go back to the 20".
2011 Overland, Dk Charcoal Pearl, Black/Saddle w/Summit Wood Trim Swap, V6, QL, 20" wheels, 730N