[MNM]

I have a 2008 Pro 4x and I had to have the Transmission rebuilt. I noticed when I picked up the truck from the transmission shop that it was idling horribly when I had it in gear. When I have my foot on the brake and I shift it into neutral it idles fine, once I drop it in gear the RPM's drop below 500 and an ugly metal on metal banging noise starts. As well it feels like it just wants to take off and I have to keep my foot pretty firm on the brake or it will take off. Once I am moving it drives and shifts relatively fine. I have taken to my mechanic and he put on the computer and he says from a code perspective everything looks good. So I have taken it back to the Transmission shop twice and he says it has nothing to do with the rebuilt transmission. Does anyone have any suggestions on what could be causing this issue when I am in gear??