99 Suburban 2500 Help

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Chad_MT

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Hey all, long time lurker, first time poster. I have a few issues that have been making me pull my hair out that I can't seem to track down. This is my 4th gmt400 and I have never had these issues before.
The vehicle in question is a 99 k2500 suburban 7.4 auto with 72,000 miles on it. The problem I will address here is a vibration I get at highway speed, worse between 68-73 mph. Also significantly worse pulling a trailer. The vibration isn't horrible, but bad enough to rattle the dash slightly and with rattle an empty tumbler mug in the cupholder quite bad.
It feels like tire balance or wheel issue, but it isn't, 2 sets of wheels and 3 different sets of tires and no difference.
Brakes were recently done, pads replaced and rotors turned up front, new drums in back because old ones were slightly out of round. No change. It's really weird because occasionally, not every time, you will get on the brakes coming off the interstate, it will amplify the vibration severely. But only occasionally.
U-joints recently replaced in rear, no change.

Also, I think this is related to the issue, when driving on highways that are somewhat rough, lots of large cracks, I will get feedback from the bumps through the vehicle and the steering wheel. I originally thought the shocks were shot, but discovered the PO had put Bilstein shocks on it so I'm pretty confident it's not the dampening that is the problem.
Any help would be appreciated, I have scoured the internet trying to find an explanation to no avail. Thanks all.
 

RichLo

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It may be a driveshaft balance issue. You can check to see if the weight has fallen off, if it has there will be a rectangular clean spot where it used to be. If its missing you can clamp a few hose clamps around the driveshaft and have the heavy screw side where the old weight used to be.

Otherwise check if there is any play in the pinion bearing and transfer case output shaft bearing.

Edit: being a long wheel base burb, does it have a 2-piece driveshaft with a carrier support? If so, that rubber bushing is probably dry rotted
 

1998_K1500_Sub

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The vehicle in question is a 99 k2500 suburban 7.4 auto with 72,000 miles on it. The problem I will address here is a vibration I get at highway speed, worse between 68-73 mph. Also significantly worse pulling a trailer.

Is there any reason to believe the pinion angle is incorrect?

When towing is the rear bumper notably lower / suspension compressed, affecting pinion angle?

Are the springs original? Rear end original? Modifications of any sort to driveshaft / rear end / springs which might affect pinion angle?

@RichLo made good suggestions.
 
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Chad_MT

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Thanks for the reply a so far.
The weights on the driveshaft are still in place, verified when I changed u-joints. I have been considering getting the shaft balanced.
Checked the pinion bearing play, all seems well. I haven't had the cover off the diff to check it. I guess stranger things have happened, but the likelihood of a 14 bolt with low miles and little towing being the culprit seems low to me, but never know I guess.
As far as I know, this thing is all original in the drivetrain and was pampered before I got it with 56,000 miles on it. It never even pulled a trailer before I got it, I had to wire it for trailer lights when I got it. I don't pull heavy either, heaviest trailer isn't over 4000 lbs.
Pinion angle I haven't considered, was this ever a problem on these?
Load on the drivetrain seems to make it much worse, like pulling a trailer. Even the other day driving home in a headwind, I noticed it was much worse.
 

Chad_MT

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@OutlawDrifter I have visually checked the cv axles, but other than that nothing. I have been suspect there as well. Any good ways to check over the front end and CV axles?
 

RichLo

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Kind of shooting in the dark now but How about your motor/trans/t-case mounts? Also rear suspension bushings. Since its worse under load something in the drivetrain has to be moving around.

CV axles, just check if the boots, if they arent cracked and spraying grease everywhere you should be fine with that mileage.
 

Chad_MT

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@RichLo yeah, CV boots are good. Mounts seem to be good, visually they look fine. I'm totally taking shots in the dark at this point too.

Another thing I have been suspect of is the transfer case. It is the autotrak case and I changed the fluid a few weeks ago and it had atf in it. At first, proper fluid seemed to help but I think it was placebo effect, I wanted that to be the fix. But any chance the atf in the autotrak cause any of this?
 

OutlawDrifter

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Another thing I have been suspect of is the transfer case. It is the autotrak case and I changed the fluid a few weeks ago and it had atf in it. At first, proper fluid seemed to help but I think it was placebo effect, I wanted that to be the fix. But any chance the atf in the autotrak cause any of this?

I replaced mine a while back and it had ATF in it as well...turns out, that blue stuff you put back in is just blue ATF. That shouldn't be the issue. AmsOil went after GM for years to get the spec on the fluid...and they(GM) kept dragging their feet.
 
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