My RPM's jump when I slow down to a stop and the tranny downshifts, its really noticable with the intake and exhaust sounds but im not sure what the problem with yours would be.
Well it's normal for rpms to move when pressing the brake right? AND you are pressing the brake when putting it in park so...Do this...put it in park and pump your brake. If the rpms go up or down like what your describing I think that's normal...
Completely different vehicle but when i had my Oldsmobile, i had the same issue with it with the weird RPMs. It sounded like the engine was also struggling to stay on when idle too. It turned out to be a bad PCV valve and partially clogged breather tubes.
Same went for my buddies 85 Cherokee back in the day, clogged breather lines and a clogged CCV system (older jeep system similar to PCV). Once these systems were cleaned and appropriate parts replaced, the engine ran smooth and didnt sound like it was gonna die when the rpms were dipping.
It might not be the issue but it is worth a shot...similar issue to what i had resulted from this. I hope this helps man.
is it the 4.7? mine jumped alot when the TPS was going bad, replace it with a dealer one before it goes bad, mine went bad on the highway going 80, dropped the tranny into 2nd (i think), scared the shit out of me, check engine light and red line, ran my codes and it was then "TPS sensor low or no voltage" thing