Re: more questions....
You can tell what axle your car has in it by the door sticker. It should be on the end of the driver's door. Under where it says "AX", there'll be a letter or number that can be translated into the ratio and whether it's open or posi. I don't have the information on what letters or numbers represent what for a car as new as yours (I think they've changed, though I'm not positive), but if you have a service manual for the car, they should be listed in there.
1989 Ford Crown Victoria Police Interceptor
2bbl Carbureted 351W V8, ported and polished E7TE heads, Edelbrock cam
AOD Transmission with B&M Shift Kit
3.55:1 Positraction Rear Axle
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