Slipping Trans

Sounds like the typical torque converter drain back problem Mopars are famous for. Like slantsixdan said as soon as you start it up shift it into neutral to let the torque converter fill up (no fluid flows in park therefore it won't fill the converter in park no matter how long it runs). It will fill up in 15-20 seconds. Then when you shift it into reverse it should work fine. I have seen numerous torqueflites do exactly this.