ChuckH is absolutely right.
Relying on Park on an incline puts alot of BAD stress on the Auto Transmission. Switching from Reverse to Drive while still rolling causes similar damage to the tranny.

That loud clunk is NOT healthy!

It wont actually ruin your tranny immediately but after time the gears will begin to slip in an out of what they're supposed to be in.

When parking on an incline engage the parking brake before you shift to Park, then release the foot brakes.

When backing up come to a complete stop, shift to drive, wait about 1 full second, then press the gas to procede.

The only time I can think of throwing the gears from drive to reverse is when you're stuck and need to rock the Xterra front to back to get out. And the only reason it's OK then is because its a rare occurrence and shouldn't be done often.