Looks like you've eliminated the couple things I would have thought of first, bearing or tire that may be out of balance. When you bought it, were there any service records that came with? If the rear end hasn't been serviced, now might be a good time to have that done where you can see the condition of the fluid. If it's still in good shape, it may just be low but if there's any metal shavings there, that's cause for concern.
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My 2007 Jeep Compass Sport, Bright Silver Metallic, 26E, CVT II, Driver Convenience Group, Sunroof, Trailer Tow Prep Group, 4x2
1991 XJ Cherokee Sport 4X4 4.0 H/O I6
2008 Dodge Ram 1500 ST, SWB Reg Cab, Bright Silver Metallic, Trailer Tow Package
JEEP - Just Enjoy Everything, Period.....