If you are switching to an open driveline axel there are a few things to keep in mind, and tricks too... Read all the threads you can before you get started. here's what I learned doing mine.
First- Bolt pattern- Wanna keep the front rims the same as the back? look for a 6 lug axel, such as those used on the Chevy Luv. Planning on upgrading to discs in the front one day, the 5 bolt on 4 3/4" circle kits are cheaper than any other.
Second- Width of axel- Keeping the same rims, use the same width. Using different rims? Make sure and take into account the backspacing on the new rims when you are calculating the axel width because the older rims had the mount surface about in the middle of the rim, newer rims have them further towards the outside which means you need a slightly wider axel.
Third- Gear ratio- Whether you are keeping the stock tranny or switching to a T-5 or some other tranny, you should figure out what gear ratio will work best for you before you go shopping for rear differentials, so you find what you need- Personally, I like 3.42:1
Fourth- Positraction? There's lots of them out there under sedans, station wagons, cadillacs, trucks and retired police cars (ie caprices). If you aren't in a rush, might as well find one.
Fifth- Brakes- While hunting through the bone yard for your rear diff, check the brakes- One of two otherwise equal candidates might get ruled out on the basis of the state of the brakes or the choice between 12" drums and 10" drums.
Sixth- Coil vs. Leafspring? Most of the 10 bolt posi's I found came with coil springs. NO big deal, just cut of the mounts with an angle grinder and cut off wheel and weld in some spring perches right where you need them. Trying to find an axel with spring perches in the right spot is a lot of unneccesary work. Alternatively, bolt/weld in a four link suspension kit and go with coil springs- Not a huge task or very expensive either. In fact, in my case it would have been at least $100 less than getting my leaf springs re-arched and repaired.
Seventh- Sway bar- Is there one on the axel you are buying at the junk yard? If so, don't forget to buy it too. They're easy to hook up to your truck and will make a noticable difference in the handling for the cost of readily available mounts attached to the swaybar that came stock with your axel. (Autozone-$20-$30).
Eigth- Driveshaft- Measure the length you need once the axel is in the car and find one at the boneyard, have new bearings put in and balance it.
Ninth- Pinion Angle. Make sure the new rear diff is set at the same angle as your engine.
Tenth- Axel/Pinion Seal- Your 'new' axel might have been sitting in the boneyard for a long time- Might as well replace the axel, pinion and cover seals- They aren't expensive at $5/each its around $20 total. You'll probably want to change the fluid anyhow so this is the time to pull the axels and replace the seals (and maybe the bearings too.) Changing the Pinion seal probably requires an Inch/Pound torque wrench so if you don't have one and it ain't broke...
Eleventh- Last but certainly not least- Emergency Brake Cables- Your new axel probably doesn't have the right e-brake cable to work with you stock set up, and as far as I can tell, no-one makes a bolt in replacement except Lokar (and they ain't cheap). However, you might do two things First, using the cables on the axel have a threaded bolt end swerged on. Second, use a wire clamp, make a loop through a 5/16th fine thread bolt and clamp down.
