I have a whining sound seems to be coming from the back. The sound is a whining noise that increases with speed and doesn't seem to change when braking. Almost sounds like a low tire pressure but ruled out tire s by switching them around and subbing in the spare. The sound does not change with transmission speed but with vehicle speed. I can't get it to make any noise while on jack stands even at 50 mpg tire spin. What do you think?

Sounds like a pinion bearing is going bad

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05 CRD; Rebuilt top end w/ARP studs, EGR delete, SEGR, 3.7L HD cooling fan, Samco hoses, in tank pump, JBA coil overs & control arms, KK clevis fork, 1/2" cradle drop, 1/4" motor mount lift, WJ rear upper trilink w/WW mount, WJ lower control arms, 4:10 gear, front/rear air lockers, ARB bumper, Rock Lizard sliders/roof rack, Mod T-stat