l have a 93 crown vic and wanna make the rear end posi what do l need and can l do it myself...
ok..so l called a ring and pinion shop and they said $480 installed..is that a good price on parts and labor?..if they are just goin to replace my gears..where can l get the same thing as them and do it myself...

richmond gear makes an awesome ring and pinion set that has a locked spool see richmondgear.com

First, r u sure the car does not already have a posi? Check the tag on the axle cover. It will tell you. The ratio is usually the last number listed on the tag, like 345. If it says 3L45 then it is already a locker.
Your best bet is to find a complete axle from a salvage yard with the same ratio & have it swapped out, especially if u have never done this type of work before.
To change to a posi, u will have to get a complete posi differential (carrier). It is also recommended that u replace the bearings at this time. A manual. You will need special tools in additon to the usual sockets. Pinion depth & differential preload gages, dial indicator, bearing removal tool, bearing press. You do not want to do this laying on the ground, so access to a hoist.

There r two ways to do this. First (probably the easiest if u have never done any work on the internals of an axle) would be to go to the junkyard & get a complete rear axle from a police crown vic or another one that has a posi. You can check this by the code on the door, or climb under the car & look at the tag on the axle cover. It should say 8.8 (that is the ring gear size) & then the ratio (2 73) or (3 08) or (3 27). Those r the only ratios they offered in ur car. If it is an open (one wheel peel) the ratio will look like the above numbers. If it is a posi (traction lok as ford calls it) it will read (2 L 73) or (3 L 08) or (3 L 27) u get the idea, the L stands for locking axle. Also keep in mind two things... if u change axle ratios it will thow off ur speedo reading & also the clutch packs inside a posi do wear out over time, so it is possible to find one in the junkyard with worn out clutches that will act just like ur open rear end, u can check by jacking up one wheel & leaving the other on the ground, then try to turn the wheel off the ground with a wrench on the lug nuts. IF it turns really easy, then u know it is worn, if it takes a bit of effort to turn it, u r probably ok. Keep in mind too, if u go this route, u will have to bleed the brakes as u r going to disconnect the hydraulic line to the rear axle. The other option is to buy a new posi unit (from summit or jegs, then swap that with the one that is in ur car. Auburn makes a decent one that would work fine for you. Stay away from anything like a Detroit Locker, u wo not like that for the street. Tell ur salesperson that u want something comparable to what came factory in the cars. This way requires pulling the cover from the axle, removing both axle shafts, then removing the main caps that hold the ring gear & differential in place, pulling the ring gear & diff, then remove the ring gear from the diff & put in on ur new posi diff. then put it all back together. You could also find a used mustang diff (let is say 86-93 or even later 90s) on ebay or a swap meet & use that. The only caution is u need to make sure it is not worn out & that it is a car diff. (cars have 28 tooth axles & trucks have 31). You can rebuild the ford traction lok posi with a new clutch pack from the ford dealer too, but they r not cheap. If u do go this route, make sure u either read ur shop manual on how to remove & install the diff. Mark everything & put it back the way it came apart (do not mix up the caps, shims, etc)

You just need a locking rear differential. It has nothing to do with ur ring & pinion. Go with a stock-style Eaton trac-lok clutch type limited slip. This is the best for u application as a street vehicle. You would be best suited having a shop install this for you. The recomendation of getting a junkyard rear is also a very good option, but it is a used part & may not be cost effective in the long run as the clutch packs in the rear diff do wear out. If ur car came with a limited slip, there is a good chance just the clutch pack needs to be replaced, which is much cheaper at the shop. good luck.

Changing rear-end gears is not something you can do at home. $480 sounds very reasonable .