[ImperialCrown]

Welcome to Allpar. If you have lost Park and the vehicle rolls, then the problem would have to be somewhere in the transfer gear, transfer shaft, differential or axles area. It sounds mechanical and not anything electrical or hydraulic. If you still have Reverse, then something must still be connected?
Can you pull the differential cover for a look? You have the A-604 (41TE) transaxle.

http://www.shinseiauto.com/japanese/transtar/42rle.pdf

 

[ImperialCrown]

Is the shift cable adjusted correctly? It starts OK in Park and Neutral?
The Park pawl and sprag mechanism are above the valve body at the far driver's side end of the transaxle. It must fully engage (spring loaded) into the cogs in order to firmly hold it in Park.
The shift lever shaft has to moved to one 'click' past 1st gear in order to place the Park rod in its holster. The valve body can then come down. Maybe you can see something wrong up in there?

 

[ImperialCrown]

Two Reverse ranges are bad. Reverse gear is strictly hydraulic or mechanical. All of the electricals can fall off the transaxle, but Reverse, Park, Neutral and 2nd gear should all still work properly. The feeling of engagement in Park is also bad. A clutch element (Reverse?) may still be applied.
If the external shift linkage is OK, then it has to be internal. Does it still start in Park and Neutral and do the reverse lights come on in Reverse?
Reverse could be a valve body, solenoid pack, internal hydraulic leakage or mechanical clutch seizure issue.
Was there any transaxle work or history leading up to this? If there are no visual clues so far, you may want to do some hydraulic pressure diagnostics.
This transaxle may need replacement if it is original and at fault.
There have been so many hardware and software upgrades since 1991, that a reman may be the best way to go if it can be proved to be an internal issue where the transaxle would have to come out.
We declined rebuilding anything this obsolete at the dealer as a reman (with the latest hardware upgrades already included) was the best solution. You would want a finned TCM with the latest software upgrade installed and make sure that the ATF cooler was clean and clear.