How might I go about converting my 99 Chevy Suburban 1500 to four wheel drive?  

Go out and buy a 4x4 suburban. Unless youre a good wrench and have deep pockets don't even try it. The list is very long of things that you'll need. Trans, transfer case, axle, springs, shocks, drive shaft, crossover steering, and all the minor things add up. My advise buy a locker for the rear and a winch and go have fun.