mine does the same thing. I didn't notice the gear whine until after I downsized to some 285 all terrains. It's rather loud under mild to moderate throttle. it gets progressively quieter as more throttle is applied and goes away completely if i lift off.
Something tells me the carrier and/or the pinion bearings are not carrying enough preload which allows the gears to change alignment ever so slightly so that the pitch changes to an audible frequency under certain speeds and drive train loads. It's getting worse and I am starting to wonder if a V8 is too much for the stock rear axle. I am part time now so there is no longer a split of the torque that would be shared in a full time truck.
1994 FZJ80. Elocked, MAF 3/2 Lift, BFG 315 KM2s, stock 4.10s, ARB bullbar and a Warn 8074 winch.
powered by an all-aluminum 335hp L33 Vortec V8
2004 Kona Stinky dee-lux.