Toyota Repair: Vibration, wheel bearings, mud tires

Question
I have a 1994 Toyota Pickup, manual transmission, V6, 4wd (auto-lockers), STD cab, 173,000 mi.

I can feel a vibration at around 15mph and 45mph.  I thought it could be the rear differential gear oil being low, but I changed the oil and it was the same.  I did the transfer case and transmission at the same time.  I also had the tires balanced and rotated.  I also inspected the brakes.

At 45 mph I can hear a sound that sounds like I have large mud tires on my truck, but if I roll down the window I don't hear anything.  The vibration has nothing to do with the rpms, just the speed that I travel.  Above 45, I can still tell there is something going on, but it's not as bad.

Could it be U-joints?  Something with the front differential?  Wheel bearings?  I'm at a loss here.  Thanks for your help.

Answer
Wheel bearings and axle bearings almost never cause a vibration but they will cause a lot of noise, since there is a vibration associated with the noise I would recommend looking at the driveshaft for the probable cause, check the U-joints carefully even if it means removing the driveshaft to check them, check them for any signs of binding up or excessive looseness.