Will not stay in park on a incline. Popping sound from trans

[jmillsUT28]

Restoring this 72 blazer and when I park on a incline with the front downhill it will not stay in park. This is a auto transmission. When it moves in park the gear selector at the steering wheel does not move but you do here a clicking noise from the trans. It does not do this when parked on a hill with the rear downhill. Just when the front is downhill.

The bushing on the shift linkage coming down from the steering column is a little worn but does not seem to be casing this problem.

Could this be a internal problem with the trans? Any other options?

Thanks for the help like always