ok my buddie just had his 4l60 built and its in a 2000 chevy 4wd ext cab. got it all done and the tranny takes off in secound and slips. so we pulled the tranny out and took it back to the place that did it they said the tranny was fine so we had the tourqe converter tested and it was fine put it back in and it did the same thing. so we took it to a diffrent tranny shop and its been there for the past 6 days. they thought it was the valve body but it wasnt that. I guess what happend is it is shifting backwards so it takes off in 4th and shifts into 3rd. they have no idea whats wrong why it is doing that. do you guys have any idea maybe somthing with computer?????????