my 95 gmc extended cab 4x4 moves in reverse but not in drive whats wrong

It could be one of a couple things. It could be low tranny fluid, which if it has a transmission fliud cooler could be from a leaking line or a leak in the cooler itself. Check your fluid level, there should be a dip stick under the hood. It could be that your transmission is toast, Ive heard of it before that reverse works when everything else wont and vice versa when a tranny goes. Hopefully this is not the case, but it is possible. Is there any play in your shifter? If there is it could be as simple as the linkage.