2000 Silverado 5.3L 2wd.....has about 89000 miles on it, pulled a pretty big trailer before I got it (maybe pulled for 5000 miles or so)......anyway, the 1-2 shift is really bad, like a bad slip, and 2-3 feels like it slips also, just not as bad. Strange thing is, truck is usually garaged, left it out one night and took off from a cold start the next morning--shifted perfectly on the 1-2 on the first shift of the morning--went back to the "slip" thereafter. Shift is worse the harder you're on the go pedal.....is it done or would a filter/fluid change help? BTW, truck is stock-no mods whatsoever.