I'm no expert - that's for sure.  I'm surprised no one else has chimed in.

My only guess is a bad U-Joint.  I know that you can get a very pronounce "CLUNKing" sound when putting an older automatic into reverse when there is a bad Universal Joint.

If those are good I have no other advise.

What may be the cause of a hard shift (slam) when shifting from park to reverse? All other shifting appears good. If I shift from park to drive, I have no issues or if I shift from park to drive then to reverse, no issues. The idle seems to be set somewhat low but ok. Fluid level shows to be okay. This is on a 1956 Ford Crown Vic with 292 and water cooled fordomatic.

Danny