I doubt any Grand Marquis versions had the Taxi package. What you want is the 2.73:1 axle gears out of the taxi version of the Crown Vic. The easiest way is to swap the entire axle (unless you have rear air suspension, in which case, yeah, you're going to have to either take the axle apart and switch gears, or chop the air suspension out of the Marquis de Sade and switch to springs. And, if you have air suspension, you're going to have to replace or repair it eventually and it isn't cheap. Anyway, the 2.73:1 gears will turn in about 30 mpg on the highway and the car will run quieter to boot.
Fastest way to spot a Taxi version is to look for "P71" in the VIN on the dash through the windshield. When you find one, then you can crawl under it and read the numbers off of the sheet metal tag attached to one of the differential cover bolts. You're looking for "73" or "273", depending on the year. P71s are taxis, but quite a few of them got used as police cars by more cost-conscious departments.