Mike - From your description you have the stock 3/4 ton driveline. Front portion of the driveshaft is enclosed, rear portion is open. To upgrade your gears, you need the pumpkin (center section) from a 64-72 with a ten bolt full-floating differential with 4.10 gears. The swap is fairly easy, but it does take two people, due to the weight of the pumpkin and the location. Follow the instructions from Kook, and you'll be okay. The most difficult part is locating a differential.
Do you have a factory service manual? If not, you should get one from one of the vendors, cost is about $30. It will be the best tool that you can buy, has instructions on how to repair almost anything on these trucks.
