Oops, not a Ford guy, but I do have a Crown Vic rear. BTT for Squirrell or somebody who knows for sure.
See if it's on the door tag, if so call the parts dept. of your local dealer. My '83 Interceptor has 8.8.
just look at the damn thing, does it have a bolt on rear cover? that means it's not an 8 or 9" if it does not have a bolt on rear cover then it's an 8 or 9, look at the lower two studs if you can get to them with a deepwell socket then it's an 8" if not it's 9"