What about the transfer case fluid, was it changed and if so did you use Autotrac (or RP Synchromax) fluid? - The Transfer case can create a vibration or shake when the wrong fluid is installed, badly needs replacing or the level is low. Many mechanics try to use DEX3/6, which doesn't have the friction modifiers that the Autotrac spec calls for. W/o those friction modifiers, the clutch plates partially engage causing a vibration or shake.
I only mention it as you mentioned that you did change the axle fluids, figuring you may have also done the transfer case as well.
Borla Exhaust, CAI, e-fans, Wester's Tune, MarkMc's lift & Z71 Shocks!