Idle Rpm High in Park, Low in Drive

Dart,
You are on the right track. Post #3 is bad advice.
The HUGE jump in idle rpm when you used MVA was because the engine is making more hp [ the rpm increase ] & the engine liked it [ smoother idle ]. With PVA, Chrysler missed the boat....Simple as that, didn't happen often, but happened this time with MVA...
Keep MVA. You will need some timing marks to dial in MVA accurately.
With MVA, the engine is already making the extra hp to carry the load as the throttle is opened off idle. That is why many off idle flat spots are cured by swapping from PVA to MVA. With PVA, the extra power is not there to take the load, it takes some time & in that time you get a flat spot.
To check how much idle timing the engine wants, disconnect & plug VA. Engine idling, in gear, rotate dist slowly to advance timing until highest rpm is reached; now check what the timing is & that is the timing reqd. Say it is 32*. Idle timing can be any combination of MVA + initial.
Depending on the above test, you may need to limit the amount of vac adv. [ fabricate a stop ]
The large rpm drop going into gear is most likely from using a fixed factory unit or incorrectly adjusted adj unit that does not work with reduced vacuum. You need an adj VA unit & screw the Allen Key fully CW [ softest setting ]as a starting point. You will then have stable idle in or out of gear.