98 Tahoe -- "Shudders under load"

I have a 98 Tahoe, 2WD, automatic transmission, 145,000 miles. I live in the hill country west of Austin, Texas.
When I am going up some of the steeper inclines, the truck does not
downshift from overdrive, and it begins to shake and jerk. I used to be able to manually downshift to 3rd and the truck would then pull the incline ok, but not anymore.
My mechanic describes it as shuddering under load and has determined that the number 4 cylinder misses under load, but not under normal driving conditions. He doesnt know what to do.
Any suggestions?
Thanks, Steve
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This could also be a trans problem, have it scanned, usually it will have a misfire code if it is the engine, also scan for trans codes with a trans capable scanner.

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sounds like transmission slippage to me.... Pull the dipstick for the trans, and smell it. If it smells like sewerage, it is burnt trans fluid, which means the clutch packs in the trans are probably slipping enough to get really hot. Number 4 cyl misfiring would not cause this particular problem, by the way...

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Thanks all for the replies!
This problem began a year ago. I immediately took the Tahoe to the trans shop, and they said yep, needs to have trans rebuilt, so thats what we did.
Within six months the problem was back so I went back to the trans shop. They said there was some adjustment problem with the pressure plate, so they fixed that.
Within three months the problem was back. This time I changed my driving routes to avoid steep inclines and learned to manually downshift at the appropriate time. After about three months of this, the Tahoe would shudder even when I manually downshifted.
So, back to the trans shop. The trans fluid is full and red. The trans guys said couldnt see a trans problem this time, but suspected an engine problem. When I first took the car to my general mechanic, he said he would look it over. After a couple of days, he agreed it wasnt a trans problem and said he had a check engine light and codes saying it was a misfire. Then after a couple of more days, he said since it didnt misfire when not under load, he didnt know what to do about it.
First time anyone has mentioned clutch packs. I'll check into this.
Thanks again.
Zombie Wolf wrote:

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Misfire under load is often bad plug wires or bad plugs.
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