If they are ford motorsport gears and you guys have some basic lifting tools and a good fighting spirit, you can do it yourself. With any other gearset, you'll need to determine backlash and that's no fun at all!
First, detach the driveshaft and with the car on the ground and the parking brake set really hard, get a 15/16(?) socket and a 2 foot breaker bar and a scissors jack. Use the jack to push upward on the breaker bar and get the pinion nut started. Remove the pinion nut. Then, you need to jack up the entire back of the car so the axle is drooping at maximum extension, drop the rear swaybar, remove the axle cover, rotate the wheels until you see this little bolt in the differential. Pull that out, spin the differential around, pull out the s-spring, push the wheels inward, remove the c-clip on either axle, push the axles out. Unbolt the clamps holding in the differential. Rotate the differential while pushing upward on it till the differential falls out (catch it!). Save the shims and remember which side they came from!! With FMS gears, you just re-use all the shims. Get a dead blow mallet and smack the pinion so it comes out. Ubolt the ring gear from the diff and put a screwdriver into the bolt holes and hit it with a hammer in a crisscross pattern till the ringgear comes off. Bake your new ring gear in the oven so it expands then drop it onto the diff. Try to line up the bolt holes while its hot. Get your new pinion and replace the old crush sleeve with a new one (usually gearsets come with a crush sleeve). Now is also a stellar time to replace the bearings on either side of the differential. Now, I happened to use my existing crush sleeve but this is what you're not supposed to do! I was lucky and got away with it for 2 years and 15,000 miles. Your choice. You'll need to be hercules and put 400 ft-lbs or so of pressure to get the crush sleeve started. Basically, you dont want the pinion to have any free play on it and just a little bit of drag (a tiny amount....25 in-pds I think). Tighten it, take it for a drive and let off the gas. If it makes any noise on decel, get down there and tighten some more.
Have fun!
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91 GT Carb conversion, holley 600 double pumper, edelbrock performer intake, FMS "C" drop springs, march 1000 underdrive pullies, crane 1.7 roller rockers, GT-40P headers, bassani x pipe, american thunder catback, FMS 4.56's, msd aL6, trunkmount battery, A/C eliminator kit, 3000 stall tci streetfighter, AOD with transgo kit, A+ servo, 300M hardened lockup shaft, kevlar bands and 28,000 gvw trans cooler, 3 core radiator, 300 lbs stripped with a full interior
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