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.