Why has my 1991 GMC Sierra 1500 stopped shifting into 4 wheel drive?

Our truck has had trouble shifting into 4 wheel drive for a while, but just recently it quit entirely. When we bought the truck used, the previous owner said there was a part in the transfer that if replaced, would solve the problem. Does anyone know what part this might be, or another possible solution, with the winter starting up, 4 wheel drive would be very useful here in michigan.|||Does your manual shift lever go into 4H, and if so does you 4 wheel light come on? if it shifts to 4h but the light doesn't come on its the actuator in the front diff. I just had mine go out on me last week I have a 1990. The actuator is about $90 from summit or autozone and is a 5 minute fix if you want to start there|||could be bad actuator on front diff, connectors unplugged at front diff, or bad switch at transfer case|||It could be your wiring from the shifter to the front axel.. . or the entire transfer case has to be changed