Why are my gears grinding?

I have a 2000 Grand Jeep Cherokee, with an automatic transmission, I have noticed recently that the gear grinds only when I am changing from reverse into drive or when I stop and go from drive into park. It never grinds when I am in drive and it is manually changing gears. Just thought I would ask if anyone had some suggestions. I like to know what I am talking about when I take it in to be looked at. Thanks