How to make a 1987 Chevy R10 rear wheel drive truck a four wheel drive truck?

The entire drive-train minus the engine, plus the entire suspension would need to be changed. Tires would also need changing. This is possible, but a very daunting task. You would need to have a donor truck in order the acquire all the parts necessary. Otherwise the cost would be in the thousands of dollars. I would suggest you sell the 2wd truck you have and purchase a 4wd.