6.0caddy

So i get my tranny in and drive it around and everthing is fine stop check the fluid everything ok. I drive out to my gate and kinda get on it and when I slow down to the gate and stop. Now i give it gas and nothing its like its in nutral. I try reverse and it grind a little the nothing. So i put it 4WD and it kicks in gear. I take in out of 4 WD and it works fine? what the hell.