Chevrolet Repair: 1999 silverado 2500 hd 6.0L, wheel bearings, drive shaft

Question
i have a 1999 silverado 2500 hd with the automatic transmission and push button four wheel drive i recently purchased. the previous owner had a snow plow setup on it which he removed before i bought it. my question is i have a bad vibration at high speeds starting at about 45 and getting worse at higher speeds. the previous owner had a new front drive shaft put on, the  yolk replaced on the front differential,all new u joints both axles, both axles balanced and the transfer case gone through all by the local cheverolet dealership. I have replaced the tires and still a vibration. i unhooked the front drive and it quits.  i tried pushing the four high and four low buttons and it sounds like it is engaging and disengaging. any help would be appreciated .

   thanks for taking the time  Jamie

Answer
Hi Jamie, Well if you remove the front driveshaft and the vib goes away then the problem must be either.....The driveshaft,or the ring and pinion in the front end. Both axles are aways spinning when driving so this is out, along with tires,rotors or wheel bearings.

                Mark