Transmission slips when it's cold out

My truck is a 2wd ram 1500 reg cab.when it's cold
out I let it warm up before I even go to drive it and
even after letting it warm up sometimes it'll go into
drive fine but once I stop and put it in park and go
to drive again ill go to back up it'll rev high not go
anywhere then catch and wanna stall out.the other
day I went to the store right down the road and
pulled out onto the road and it slipped out of gear
and reved high and wouldn't go anywhere then
caught and was fine getting home...What could
that be?