Drive train noise in Jeep Liberty 4x4

I recently acquired a 2005 Jeep Liberty, 3.7 engine 4x4. Mileage is
going on 80k. I thought the noise I heard on dry pavement was due to
worn rear tires but tire change didn't affect the noise. You notice a
thumping noise at about 20 mph that increases in frequency and turns
into a whine at 45mph. At 60mph the whine is mostly not a problem.
I guess it's a bearing somewhere? Any advice? How costly to fix?
I do not thing it is a front wheel bearing as turns don't affect the
noise. The noise doesn't change with engine rpm, only speed.

 

Sounds like a u-joint. They go out. Not expensive and not that hard
for a DIY'er. Search for a how-to, there's lot's of them out there.