20 mph shudder -- any known causes for this?

Accelerating, slowing down, going a constant 20mph, doesn't seem to matter. I thought tranny when I first felt it, but doesn't seem likely since it does in when not under load as well.

Had a local indy shop owner drive it, he said he's been working on cars since '78, and this doesn't really feel like anything he can put his finger on, His WAG was definitely driveline, maybe motor or tranny mounts.

Put it up in the air and thought maybe it was the tranny mount, but pulled it out and it looks fine.

I have had this issue also it begin around end of summer last year. then just went away...

Sight unseen it sounds like a u-joint. Check them first.

Don't just check visibly, or by mving by hand with the driveshafts still on truck, Actually remove the driveshaft, and articulate them throughout the entire range of motion. I think you'll find that you've got a tight one.

Hope this helps.

Have your tires checked and balanced.

Also check them for a bad tire belt.

I was poking around looking for info on a new project truck and read about your shudder.

My daily driver is a '98 S-10 Extended cab. It does the very same thing, shaking at 20-25 MPH in any gear. At highway speed the shake is still there, very slight, but enough to make the rearview mirror blurry.

I've got new tires, and eliminated a ton of shakes, rattles, and buzzes by replacing all three u-joints. Two were absolute toast! It was smooth as glass for about 120 miles, then the shake came back. Nothing like before when the joints were practically falling out, but it is there.

I've been told the center hanger bearing might be worn from the bad joints. I'm going to have it changed, but wonder if the clutch might have something to do with the vibration, too.

I went over to the local shop to have the center bearing changed and they also found the tailshaft bushing was worn. Simple fix, and no more shakes!

GMC says they back their products, but i have a GMC built transmission that was had a gaurentee of 3 years or 60000 kms. their new warrenty is for 90000 kms.

i have just over 20000 kms on my trans and it kicked it. i maintain it as i should, and drive it in normal expected conditions.

i have contacted my local GMC dealer and they told me that they would not stand behind their work or their warrenty.

I recommend to all not to buy GMC products of any kind, they will leave you high and dry, with an empty wallet.

ps - No one told me GMC stood for Gimmie More Cash

2003 Sonoma Ext cab 4.3 L
Returning from florida last week I notice a vibration from the drive shaft
that kept increasing as the miles went by. I made a stop at a garage in
SC to have the U-joints check . The rear joint had a bit of play in it, and
told them to replace it only to find out that no U-joint replacement existed
for that model and I had to replace the complete drive shaft at a cost of
$ 1200.00 + labor and I had to wait for the parts 2-3 days???. Back on the
road with the vidration that got to a point that I was afraid the joint might
break made a stop at a GMC dealer in VA to be told the sama story that
the U-joints are not servicable and and to replace the complete drive shaft
for a total cost $ 1611.00 that what I call profitable engineering ???
Pls let my have yrs comments

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