Only reverse

Anybody hear anything about starting off and manually shifting an automatic. Would not **** itself. Then get to a light and it doesn't want to have any forward gears. Only reverse. Fluid is not bad and does not smell bad. Few told me electrical and one guy told me sun gears. I'm just lost on this one. The trans was fine. I blew a 6.2 L motor up that was in the truck. I replaced it with a 6.0. since replacing the motor I have accumulated this issue. The transmission worked what seemed to be perfect before the motor thing.