Chevrolet Repair: 1999 yukon 4wd noise, front wheel bearings, drive shaft

Question
I have humming noise,that sounds like I have 33 in. mudders on the truck. It is much quiter in 2wd drive, but when I hit the button for 4wd it might as well be an amplifier switch.I have taken  it to a couple of local trans shops and heard everything from ujoints to pinion bearing.Any help with what i should check or do first would really help thanks, Mike

Answer
Hi Mike...the fact that it is quiter in 2wd is a puzzle...i would suspect the front wheel bearings, a simple drive and turning the wheel from right to left hard should tell you... if noise changes while turning side to side then i would look there. if not then check front pinion for excessive lash to the crown...does noise get any worse when you throttle up... if front end it should. another check would be to remove the front drive shaft and try it... if noise gone then i would suspect u joints....you might also check transfer case and make sure it's full of fluid....front end noise on 4x4s are hard to find...i hope this helps