Chevy silverado 4wd gear noise, thumping, and seize

Hello,
I have a 2005 Chevy Silverado 1500 4WD. It's got approx 39000 miles on it. Two days ago it started experiencing some trouble. There has been an
intermittent thumping noise, usually when making left turns and occasionally when making right turns. The thumping noise is accompanied by a slight vibration in the steering wheel. This occurs in 2WD and auto-4WD modes.
If I had to take a guess, I'd say the noise is coming from the front end, possibly on the drivers-side, but it's hard to localize.
I've tried jacking the vehicle up and spinning the front tires by hand, and notice nothing wrong. Verified the front diff has plenty of fluid.
When shifted into 4WD (4HI or 4LO), there is a good deal of gear noise coming from the driveline. Turning a sharp left will seize the driveline. It's possible it would move if I gave it some gas, but I didn't want to mess anything up, so I only gave it a little bit of power.
Any ideas what is wrong? I'm going to the dealership probably on Monday.
Thanks
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snipped-for-privacy@nospam.org wrote:

I'd bet you feeling the ISS thump. If still under warranty the dealer will grease it but you might as well learn how to do it now.
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Thanks Eugene,
It's 3000 miles out of warranty, but if all it needs is grease, then hopefully it won't cost too much.
I'm not familiar with the ISS (I found a few refences to "intermediate steering shaft" on the net -- is that what we're talking about?)
To further diagnose the problem, I should mention that the thumping does not seem to be coming from the act of steering itself (i.e. I can park in the parking lot and turn the steering wheel from end to end with no thump), but rather from driving while in a turn. For example, if I hold the steering wheel in a left-hand turn and drive around in circles in the parking lot then it's thumping. That's what originally made me think it's a front-end problem and not a steering problem.
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snipped-for-privacy@nospam.org wrote:

Thats it

the ISS has a slip joint so as the body flexes a bit on its rubber mounts the shaft needs to get a bit shorter and longer. The problem according to GM is there wasn't enough grease from the factory so the shaft binds up and sticks and makes the clunk. The real world answer is the grease hides a design problem for a little while but then you have to grease it again. See if you can just buy the grease kit and learn how to do it yourself and you can reuse the parts in the kit just re filling it with your own grease, the same stuff I grease the steering joints with works fine for me. Someone on a forum said they drilled and tapped the shaft and added a grease fitting so they could do it without removing the shaft from the truck. I haven't tried that yet.
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wrote:

ive had to do so many of these shaft lubes i started trying different approachs......the simplest, quickest most lasting ive found is pull the boot back from inside the cabin and spray power lube into joint, turn steering from stop to stop several times to soften and flush old lube. then spray white lithium grease (alot) and again turn from stop to stop several times........havent done the same unit twice yet......installing grease fitting was a thought (good thought) but i do so many (rental fleet) i needed a time friendly solution that didnt require shaft R&R.....BTW: we run ford, GM, and toyota in a 400 car fleet. GM is the only brand ive had to do this on. over and over and over since 2000....
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KjunRaven wrote:

I don't understand why they just don't come out with a fix for it as dealerships keep replacing the shaft and that would cost more than a real fix. At least my truck hasn't been in the shop as much as all the Toyotas owned by people in my office, they are always needing a lift and theirs never go offroad. IMHO planned or unplanned shop time is still time I'm not driving it and mine hasn't been in the shop yet. I'm amazed at the mount of time the overrates toyotas spend at the dealership.
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snipped-for-privacy@nospam.org wrote:

Don't use 4x4 on dry pavement. RTM. You don't say if this is what you are doing or not. My apologies if you are not using 4x4 on dry pavement.
Bob
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If that's all it is, then I feel a bit silly... I was so busy trying to diagnose the thumping noise in every conceivable mode of driving that I wasn't paying attention that I was driving around in a parking lot. To date I've only used the 4WD offroad, so I had no idea that's what it did if you engage the thing on dry pavement!
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snipped-for-privacy@nospam.org wrote:

Yup. If you continue to use 4x4 on dry pavement you'll really mess the truck up. I'm glad that's all it was.
Bob
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Actually it turned out to be bad bearings in the front end. The dealership said it was rare for them to go out, but it does happen every so often. They replaced them and the truck sounds normal. AFAIK, the only time I've driven it on dry pavement was when I tried to diagnose the thumping issue the other day, and that was only for a minute or so.
Eugene was right on about the diagnosis of the thumping noise in the steering. They performed the standard grease remedy via the service bulletin and that is fixed also, at least for now.
I guess what threw me is two seemingly unrelated problems in the front end developing at approximately the same time...
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