I have a 2000 Tahoe with 74,000 miles shift from park to any gear there is a clunking sound from the rear of the vehicle, also if i am driving down the road at 35 mph or so & step on the gas the tranny shifts down & I will get a clunking sound? Check u-joints & they seam tight.

There is a tsb on this problem or a simalir problem. The slip yoke has play in it. Search the silverado and seria forums.
Here I did a search check out these http://www.automotiveforums.com/vbulletin/search.php?searchid=1536532

The dealers usually say it's normal slack in the driveline and is more noticeable in 4wd versions. Take it for what it's worth, but it seems that GM doesn't think that it's problem. My 2000 new-style 2wd did it a little, but my 2003 4wd is more pronounced, particularly from drive to reverse. No mechanical issues (yet ?), so it doesn't seem to be hurting anything.