1996 Dodge Ram 4 wheel drive wont work

i bought this truck and the four wheel drive worked once i checked everything i can think of with the shift linkage and all looks good how can i tell where to go next i am lost when it comes to four wheel drives but can handle almost anything else on the truck i dont know how to tell if a vaccum leak or transfer case is bad please help i need four wheel drive on an almost daily basis where i live