Hello everyone any odeas of what could this be. My 2000 gmc yukon xl 4wd is not shifting when in third gear. When i drive on D and go up to 20 mph rpm goes up but it doesnt shift i let the gas pedal go and it makes the shifting and starts driving well but again when going up to 50 mph my truck seems to slow down. Also, if i drive it like if it was a standard set it up in 1 then 2 it drives fine but when i put i on 3rd it doesnt take it and it starts to slow down. I dont have a check engine light on so i dont know what could it be it seems like that third gear turn bad can some one help me if anybody got the same problem please share any idead or opinions are welcome