If you choose to rebuild your own rearend, Randy's Ring & Pinion put together a comprehensive 378-page book on differentials that is a great resource from how and why, identification, and modification, which is nice to have in any library whether you're building a rearend or not. Also, the installation kit instructions from Yukon describe the rebuilding process fairly well, but even being a hands-on guy, I still left this one to the pros.

Once you get your differential good to go, it is important to follow either the manufacturer's or service center's break-in procedure to ensure that all the work and your hard-earned money isn't wasted. Randy's and Unitrax's break-in sheets basically mirror each other, which is comforting. Both outline the following:

* Only use a premium national brand 80W-90 or 85W-140 GL-5 gear oil, and make sure to add the proper friction modifier fluid if you have a limited-slip or posi unit and every time you change the gear oil. Make sure the drain plug is tight.

* Drive the vehicle for 15-20 miles and then stop and let it cool for 45 minutes. Avoid aggressive starts, hard stops, and driving long distances at a steady speed during break-in.

* Repeat the previous step once. Do not tow for the first 500 miles, and do so gradually afterward. Avoid wide-open throttle acceleration for the first 300 miles.

* Change the gear oil after the first 500-2,000 miles with the same type of fluids.

Again, check with the manufacturer or service center, but following their break-in procedure should give you years of trouble-free use of your fresh rearend. Next month, we'll show you some tips for putting the rearend back in your '67-72 Chevy truck.