I have a 2000 Silverado with the V-8 and an automatic transmission (not 4-wheel-drive).
A few days ago, while in park with the engine off and on a slight incline, the truck rolled back about a foot.
If the movement was a onetime thing, it may have been a normal occurrence. There's a "hook" type metal lever (parking pawl) which fits into grooves in the transmission's Output Carrier Assembly every time you put the transmission in Park.
The output carrier is splined directly to the transmission's output shaft -- that's how it stops the truck from moving. It's like a big sprocket with a metal pin shoved in between its teeth to stop it from rotating. Sometimes you can park at just the right spot where the parking pawl is on the very edge of a tooth, then, for whatever reason, it comes off the edge and the output carrier continues to rotate until the parking pawl catches the next tooth. I never measured the distance, but it may seem the vehicle rolls almost a foot.
If there was damage or insufficient spring tension holding the parking pawl in place, you would most likely be hearing "click-click-click" while the vehicle continues to roll down the hill. With that said -- if it happened more than once and seems to be abnormal -- by all means have it checked out.
The parking pawl and the output carrier are fully visible for inspection by simply dropping the transmission's oil pan. The inspection can be done as part of a transmission service, anyway.