Sometimes, when my car is parked facing uphill, the shifter is difficult to move out of Park, and the transmission engages Reverse with a bang. The latter happens more than the former, but never when on flat ground.

It'* an 4T60-E with about 130,000 miles. The fluid and filter were changed very recently. On really hot days, I can get a P1870 code set. Regardless, it has no driveability issues and still shifts quickly.

This doesn't sound like something that I can fix, but I'd like to know what could be the problem.

When you park on a hill (uphill or downhill) engage the park brake while still in Drive with your foot on the regular brake, then shift into Park. That make clear up your problem.

Sound slike your parking prawl is holding on. It'* a pin that hold the diff in place, teh diff rolls a bit and grabs the prawl pin.