I think you need to take it to a good shop - have them do a compression check. I just bought a 2000 SLT QC 2WD w/ 5.9 and I've got to be carefull not to light the tires at the lights. I'm in the process of detailing my old truck for sale ('94 extended cab, 2WD w/ 5.2 and I have to stand on it hard to get a bark at the lights. Neither one shifts down on small hills. I have noticed that the 5.9 doesn't have as much go as I would expect in passing mode. both are 3.55 gears.

 

Question? Tires? I had 99 3.9 Dak sport 2wd ext cab auto trans Bone stock 3.55 gears with 255/55R15 tires and it would light up the tires every time and would haul *** on the highway. Now I got a 2003 3.9 Dak Quad cab 2wd auto trans heavily modified with undredrive pullys K&N filter ported and polished TB headers Magnaflow exhaust deleted fan Superchips programer 3.55 gears and 245/70R16 tires and it is a total slouch on the road compared to the stock 99 I had. The tires are 2" taller and I wouldn't be surprised allot heavier. It's the only thing I can think of but I could be wrong.