Could it be a axle linkage bearing, like the ones on the extended cab rear drive shaft? THere is one on the front IFS 4x4's as well. any chance a clutch could cause that?
How about a fan clutch or AC clutch? Even an alternator bearing? Something related to higher speeds though....any way, those are my shots in the dark.

Sold Truck: 89 DLX extra cab 4x4 22re Automatic/ New truck 97 T100 4x4 auto Sr5