Rear diff maybe?  Gears starting to whine?  I had a bent rear axle on a Suburban once that acted that way too.  Or bent rim? Slightly bent wont necessarily be out of balance enough for noticeable vibration, but noise will be present.
					
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2007 Light Khaki 4x2 Sport
 - Debadged - LTT CAI with Amsoil Filter - JVC HD Radio - Infinity BassLink
 - Iggee Seat Covers - Black Moabs - 265/75-16 Cooper Discoverer ATP - 4.10's
 - DIY Franky sitting at 23.75" - Al's 4.5 UCAs - Trimmed F&R Bumpers
 - Removed Rear Sway Bar - Removed Rear Vibration Damper
 - 75k miles so far...
