4WD vibration @35mph

[chevalanche]

First off; Happy New Year to all! I have a 2005 Avalanche 1500 Z-71 4x4 with 32K on it. I have only engauged the four wheel drive about 3 times since new (mainly to assist getting out of my driveway in the snow). Last week we had a snow storm and I decided to activate the Auto 4WD; everything worked as it should. As I approached 35mph, the truck developed a vibration throughout (not in the stering wheel); as I increased speed the vibration subsided but never totally went away. In 2WD the truck rides as smooth as glass. I immediatly thought of U-joints or the front driveshaft balance being the cause. Being that I never use 4WD and the shaft does not turn in 2WD I thought it strange that there would be any wear on the components. Today I went under the truck and checked the U-joints, shaft weights and condition of the shaft; all were solid except for a slight bit of play (1/8" +-) at the yoke where it goes into the front output shaft on the transfer case. That movement would positively cause the vibration. Is it possible that the bearing is bad even though it has near zero miles on it? I have been doing research on this and have come across posts where fitment issues between the splined output shaft and the yoke cause a wobble and in fact, the splined shaft was not loose in the transfer case. Has anybody on this site ever experienced this with their GMT800 truck? I was even thinking maybe GM has issued a TSB on this. I would appreciate any and all comments and information. Thanks in advance to all.

 

[Donut Slayer]

U-Joints shouldn't have any play. That's where I'd look first.