?Drivetrain vibration, '04 Silverado

I have started to notice an irregular vibration at about 45/55 mph. I had my front end lined up and tires balanced. The truck has 51,000 miles on it. It is still getting 19 mpg, so I don't think it's the engine (no check engine light either). I crawled up under it and tried to turn the driveshaft, and the u-joints seem ok. I had my wife get in the truck with the parking brake on, start the truck, and at idle put it in drive & reverse. The drive shaft seems to have a little bit of "slop" in it at the rear end. It would turn back and forth a fraction of a turn (maybe 1/2 or 3/4 inch on the driveshaft), and I could hear a slight "clunk" noise as the slack was taked up. I don't know if this is a normal amount of slack or not. What say the experts? I can't believe the rear end is going out already. I drive like a little old lady, and haven't even towed anything with the truck. The gear ratio is about 3.4 with an auto trans. Are there any recalls that anybody knows about? Anybody have any other ideas before I take it to the dealer? Thanks.

I think that you will find your u joints are shot. I changed my first ones on my 02 Silverado at 46,500 miles and 115,000 miles on the second .Take them apart you will see.