99 2500 Engine Buck (shutter)

Gentlemen,
I have a 99, 2500, 4x4, auto, 6.0L with 64,000 miles. The problem that I am experiencing is that at approximately 45 mph, transmission in 4th, going up a hill, the engine will start to buck. If I manually down shift it to 3rd the problem goes away, or if I hit the gas to force the down shift to 3rd or 2nd, the problem goes away. It is very similar to leaving a manual trans in high gear when coming to a stop.
Also, sometimes when pulling out of a side street and I need quick acceleration, the same problem may occur!
Is there an adjustment or is this a computer thing? :confused:
Your help is appreciated.
Mark

torq converter, you can try a fluid change it may solve the proby or make it worst or something you can do right nows is go get a bottle of lube guard and drop it in there and that should fix the issue....do you have a 4L60E or 4L80E?

I am having a similar issue while towing a trailer with my 99 silverado 2500 6.0L. I always pull my truck down into 3rd while towing and get some shutter going up hills and such under a load. I figured it was the fuel filter. Do you think it could be a similar issue or different? Also where is this lube guard sold and what exactly is it?