Check your coolant level and your hoses. If there's an air bubble in the system or if your coolant's low, you won't get heat in your truck (won't allow coolant into the heatercore). Is your engine operating at normal temps? If not, some cardboard behind your grille will help the engine warm up better but I'm pretty you guys are only getting like 20* temps, not -40*. Your truck should run normally in 20*.
Only other thing I could think of off the top of my head is that the heater control isn't switching the duct back and forth to let the warm air into your cab. Is there proper resistance when you switch it back and forth (pretty sure '91s have cable actuated heat controls)? LR
'09 Ford Ranger FX4 that gets abused a lot... but doesn't like to swim though.