I experience the same thing with mine. I know It's the converter on mine. It even trips a code or two for it. I would scan it to double check. But roberto has the right idea. Probably the clutches in the torque converter. There really expensive from the little looking I've done. So I'm counting the days my car drives forward.
Also my car responded very well to the oil change. I used carquest ATF Dex/Merc IV I believe. I don't believe the oil has that big of an effect on things as long as It's the right type.
2003 GMC Sierra SLT 2500HD CC/SB