There are a couple of things it could be.
1) Wheel bearings. Raise the tires off the ground and check if the wheel has any play in it. I have been seeing a lot of these lately.
2) Check the tires. Sometimes a belt in the tire comes apart and can cause a vibration.
3) Brakes, Make sure all wheels turn freely.
4) Idler arm. Here is a bulletin about it:
Let me know what these things test out to be.
Some customers may comment on a shake in the steering wheel or that the front tires have wear.
This condition may be due to excessive play in the idler arm assembly.
Inspect the front end components for wear . Refer to Symptoms Suspension General Diagnosis, Steering Linkage Inspection and Idler Arm Inspection in SI. If the Idler arm is out of specification, replace the idler arm and pitman arm as a set with the new designed parts listed below. The new idler arm and pitman arm has a 3 degree angle built in. This will add in the clearance between the tie rod and the frame. Refer to Idler Arm Replacement and Pitman Arm Replacement in SI.
Idler Arm Kit
Pitman Arm Kit (3 Block Tooth Design)
Pitman Arm Kit (4 Block Tooth Design
Vibrations and noises are hard to diagnosise without being there.
Transfer Case. Check the fluid in the transfer case. I believe it takes a 18mm wrench to get the fill plug out. check for leaks. Also I found this info from GM
Also check the U-joints. They normally cause a squeak when take off.
#PIP4070A: Transfer Case Front Input Shaft Seal is Popped Out - keywords fluid grind growl inoperative leak noise - (Jan 25, 2008)
The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.
Technicians may find during any repair to the transfer case that the transfer case front input shaft seal is popped out and may be leaking. The cause may be due to a failed rear bushing and seizing of the propshaft slip yoke splines.
The front seal being popped out will be a result of this internal transfer case concern causing the slip yoke to overheat and bind resulting in the front input shaft seal popping out.
Tear down and inspect the transfer case, repair or replace the transfer case in the most cost effective way, replace the rear prop shaft slip yoke due to a possible binding concern and to prevent repeat repairs.
The transfer case fill recommendations have changed. Fill transfer case to the bottom of the fill plug. Add an additional 1.0L through the threaded speed sensor hole. The total fill amount is now approximately 2.5L.
Dan,this seems more like it as i did notice some leakage at the transfer case.(sorry)Thankyou!