new vibration

[timh975]

Hello everyone, my name is Tim, and I'm new to the forum here. I've found a wealth of info here, and hope to find out more. I own a 2000 GMC Sierra 2500LD, ext. cab short bed 4x4 NBS. I just rolled 68k miles, and started experiencing a new vibration coming from underneath, kinda like a thumping/grinding feeling. It only does it at highway speeds periodically, at even throttle, meaning it doesn't get worse speeding up or slowing down. I tried a set of my friend's new Michelin's, thinking I had a bad tire, not it. I've changed the rear u-joints, nothing. Drained the rear end fluid thinking something in the rear diff was going bad, not it neither. I was told it might be something in the front end going sour, going thru the torsion bars to where they mount under the cab. I was planning on checking the front end tommorow. Any ideas are greatly appreciated! Thank you.

[Wingnut]

You only replaced 1 u-joint? I'd do the other one first. They're only what...$10?

[MAGNUM]

Tim, if your LD has the auto trac I would have the front driveline u-joints replaced. I spent 6 months looking for a vibration on a 2000 suburban and finally changed those. There was no slop or wiggle to them but they were just dry.

[timh975]

Wingnut, I changed both u-joints on the rear driveshaft, I wouldn't just replace one. I even put the greasable u-joints in. I haven't messed with the front driveshaft yet though. Magnum, I do have the autotrac transfer case, but I don't think the front driveshaft spins as long as you're in 2WD.

[MAGNUM]

Yep, this is the only T-case I've found that the front driveline does turn in 2wd, there seems to be enough drag in the clutch pack in the T-case that it turns the driveline. Although it doesn't do much for your fuel economy, it is what allows for the auto four wheel drive to be almost instant. Look real close at the u-joints for rusty dust at the seals.