Chevrolet Repair: 2000 chevy tahoe 4 wheel drive, timken bearings, chevy tahoe

Question
I bought a 2000 Chevy Tahoe LT,5.4 V8,Auto Trans,Auto 4X4,63K miles, last year from a private individual.It now has 68K.The wheels and tires were upgraded from 16"to17" off a later model Tahoe.I noticed a humming/buzzing noise from the front end approx. 3K miles ago.Starts at approx. 45mph thru approx.65 mph where the road and wind noise get loud enough to mask the sound.The humming/buzzing noise stays the same reguardless of road surface or turnning side to side.Also a fair amount of oil was being blown out of the front diff vent tube. I have had all the bearings replaced in the front diff. with Timken bearings that were checked prior to installation.The vent tube swivel fitting in the diff was found stuck at approx. 2 o'clock position and has since been freed up.The pinion bearing and the left otb axel bearing were found questionable prior to replacement.All this and no change to the problem.
Next phase,after more investigation we found that when all four wheels are in the air,trans in neutral,engine not running,4WD in 2wheel high position,right front axle verified "not coupled",while rotating the left front axle the rear wheels turn.We believe that the clutch pack in the transfer case is not releasing,thus causing the spiders to  turn all the time heating the oil and venting it out.4WD High seems to be opperating normally.Pulled the fill plug on the trans case and found the oil at the proper level but a very dark brown, not clear blue.Obviousley the next step is fluid change.Is there anything you can add? thanks for your help...Kevin

Answer
Hi Kevin, You should be able to rotate the front wheels(either one) and the rear wheels should not turn nor should the driveshaft going to the front axle. If it does you have problems inside the front diff. The front driveshaft does turn all the time on these trucks, 2 wheel or 4 wheel. This is because of the clutches in the transfer case, normal!! Change the fluid though dark is some what normal but not good. You might want to check the front axle actuator to make sure that it releases in 2 wheel. This is controlled be the switch on the transfer case(3 wire switch). In 4 wheel it sends 12v to the front axle actuator to lock the front end.

Hope this helps

Mark