Help with my 1984 bronco?

I have a 84 ford bronco v8 302 automatic with the 4x4...The problem is that when i start the truck and put it in drive it doesn't go anywhere, i have to rev up the engine high then it will kick in to kick. It goes into reverse fine. as long as the truck is on it will go into drive with no problem..Can someone tell me what the problem is? I'm already starting to think the worst.