2004 2500 HD Silverado Rear Problem???

Had to buy a used truck (112,000 miles) because my other Silverado got totalled by a snow plow this winter. I havent driven it much due to having a gas milage beater car. Sold beater car and now driving this all the time. I have noticed when I get it up on the highway to 65-70 I get a vibration and a rough sounding noise from rear of truck. The faster I go the worse it gets. If I let off gas pedal it goes away. Today came to a red light and went to take off and turning(same time) didn't go right away it almost acted like it rear clunked and was grinding around the corner. I checked for any play in drive shaft and found nothing. I do not notice any movement in the trans/tranfer case mounts. I have towed an enclosed trailer 10000# up hills for 2hr ride each way and trans temp never got over 200 so I was leaning towards the rear. I checked fluid and see no debries or burnt color. Trans fluid seems fine. One other thing truck when you start it it has a rough idle for about 20 sec. Since I purchased truck I have changed plugs ,wires,air cleaner,cleaned MAF and throttle body and the fuel pressure regulator. Went from 11.9 to 14.3 on gas milage. I plan on doing a trans filter and fluid this wekend. I plan on camping over the 4th and hope it don't leave me sit before I can get the problem fixed. I scanned truck and get no codes.

on my 2000 LT 4wd Silverado earlier this year I had to replace the rear DRIVE SHAFT U joints at the rear diff. I had a vib around 67 MPH. checked the u joints and tires and rotated tires could not find the problem until I dropped the drive shaft. U joint was seized one end cap . no play at all , locked up solid .replaced that u joint , smooth now. mark before dropping put back in as it was.

since you just purchased this remember DO NOT USE DEXRON III... YOU MUST USE DEXRON VI .. 2006 GM NO LONGER APPROVES OF DEXRON III !

REPLACE ALL DRIVE TRAIN FLUIDS / DROP REAR DIFF COVER CLEAN and INSPECT BOX.

ok, easy and cheaper. Hope that is the problem. Thanks

ok, easy and cheaper. Hope that is the problem. Thanks

dropping the rear drive shaft is not that hard. after you do this the first time it takes 15 min to do it again. rust on the first removal and prep will take maybe an hour to do ..

I was surprised though how the u joint was rusted solid , no play at all and I got those vibs. I was expecting some tweeting noises but the drive shaft angle was zero so it did not have to move .

I had the wheels off the ground spun axles front rear could not find the problem until I dropped the rear axle.

ah yes rust.... I went and sprayed some gogo juice on the rusted bolts for an over nite soak then did in again this morning and will do a few times before I tear it down Saturday. I am going to do both u joints while I have it out. I can't believe how rusted/pitted the driveshaft is compared to my other which had an aluminum driveshaft.