1999 Dodge Ram tranny acting up

Hi, I have a 99 Ram diesel 2500. Sometimes after it has sat for a few days it doesn't want to move. I start it put it in drive and nothing happens. As if I were in neutral. I usually put it in reverse to get it moving, then it will move in drive. BUT, lately it hesitates when driving. If I start from a stopped position, I t will move then suddenly nothing. Again if I were reving it in neutral. If I let off the gas it will catch and go. It is fine as long as I don't stop. Once I am moving it is fine. But, I can hear a rattle coming from underneath that gets louder as the speed increases. Any ideas? thanks