ADVICE NEEDED '03 x5 3.0 Tranny Issues?

[Wcreech8]

Hey guys, so I a few weeks ago I was riding around after a cold winter storm hit. I was at a light and hit the gas pedal pretty hard and when I did I heard a boom underneath the car. Figured because it was so cold maybe something messed up. Got the car on a lift and saw that when the car was in drive, the rear tires would rotate. I got the advice to check the splines on the front driveshaft and the output on the transfer case so I did that and everything is fine! The transfer case chain is also fine. Another weird thing, my dad was underneath rotating the front and rear driveshafts because there was no resistance and heard a "ping" and everything was fine. Took it for a drive around my yard and then when he put it in park, the same problem occurred again. I'm stuck. Is my problem in the transmission or the transfer case? I need some help guys