1999 GMC Yukon XL. 5.7 liter V8 check engine. Just bought from used car lot. Replaced fluid and filter.
Question = when you take off in drive it takes off but then the engine revs and rpm are high but the car isn't going very fast.
SF: The e-mail address you left was incorrect so i could not contact you directly. Sounds by your description that the transmission has lost 2 gear, at least. Several components internal to the transmission can cause this. It will most likely have to come out and be disassembled.
The check engine light is probably going to have a code p1870- transmission component slipping. You can go to any large national parts store chain, and they can scan it for free. You never know, you may have some other codes not related to the transmission and can give you an idea if any other systems on this used truck are having problems.