1999 Chevy Suburban rear end klunk sound

I recently bought a 1999 Chevy Suburban K1500 LT 5.7L 4x4 66,000miles. Probably half the time when coming to a stop, when I pull out there is a clunk sound coming from the rear of the vehicle. It's just one klunk and then everything is fine until I come to another stop. I suspected a bad u joint and had the mechanic at the Chevy dealer check it and he said it was in good shape and that the sound I'm hearing is normal for Suburbans and Tahoes as the rear end is syncronizing with the transfer case. I have several friends with similar Subrubans and have never heard this sound when riding with them but they certainly hear it when riiding with me. I recently got back from a 1700 mile trip and several times the klunk sound was much louder and one time it klunked so loud and hard I thought I may have a transmission laying on the ground. Always after the initial klunk it drives just fine. A friend of mine said it sounds like a bad yoke. Is replaceing the yoke an expensive procedure? Any ideas?