First priority on the rear axle is to determine what you have. Being a GMC it is likely a Dana 44. This link will help with identification:
http://coloradok5.com/axleguide.shtmlIf you do in fact have a Dana 44, just do a Google search on “Dana 44 limited slip” and you’ll find pages of information on what’s available.
There are many aficionados of the V6. If you check in on the Jolly GMC site you’ll likely get a great deal of advice to keep the stock drive train. If you want to restore the truck, I would highly recommend joining their forum.
http://www.6066gmcguy.org/The GMC V6 truly is one of the most over engineered engines ever developed. It is a torque monster built to last for hundreds of thousands of miles. It’s also very heavy (I believe over 850 pounds) and gets rather poor gas mileage. Many owners upgrade to a 2 barrel Holley carburetor for improved performance and ease of tuning. A complete rebuild will likely cost well over $3,000 and some of the parts are hard to find.
IMHO none of these points necessarily make it a good choice for a daily driver. These issues are what convinced me to swap to a small block Chevy almost 20 years ago. Chevy parts are dirt cheap and readily available, the finished vehicle accelerates faster and gets much better mileage. And the extensive aftermarket offers many affordable options from alternator brackets to intake and exhaust manifolds.
I’d suggest before turning a wrench, spend a generous amount of time analyzing how you want to use the truck when it’s finished, and if you would gain more pleasure from a pure stock fully restored truck or a street rod. That should help decide the correct path.
