Friend of mine has a 99 Jeep Cherokee, with the V8, that has developed a whine or steady growl over 30mph. She said she first noticed it about the time she was climbing through the mountains and nearly overheated, although that may not be when it started and is unrelated.
Noise is speed, not RPM dependant. It sounds kinda like the transmission (auto) is stuck in a lower gear, but highway RPMs are nice and low. Transmission and transfer case are both full, fluid looks good, not burnt. No change in the noise when cornering. Ujoints seem ok, no slop in the driveshafts.
?!?!?!
Possible worn wheel bearing.