Re: Rear diff whine.... I think...
I still think its a hub assembly. If you haven't replaced them by now with 120,000 miles then it wouldn't be uncommon for them to go. Could be both sides too. When mine went (Just after I installed lowering springs btw) I really thought it was my front diff, try getting up to about 30mph and steer back and forth, if it's a wheel bearing transferring the weight of the Jeep off of the bearing will change the hum/whine.
2005 Jeep Grand Cherokee Limited 5.7L
PEL / Khaki-Black
22" x 9.5" Diablo G-Force Paint Matched Inserts / 17" x 9" Eagle 63 / Toyo Proxes 265/40/22 / BFG M/T 255/75/17 / SRT8 Rear Bumper / SRT8 Grille / SRT8 Black Leather Seats F+R / Magnaflow / 5" Tips / Mopar Lip / Mopar Running Boards / Weathertechs / Commander Shift Knob / K&N / Alpine Type R / Alpine PDX 4.100 / Debadge / Bilsteins / H&R 1.5" Down / Daystar 2" Up / Diablo Predator 93 PCM Tune - Performance TCM Tune / Morimoto Mini H1 Retrofit 6000k