Well, it would sure sound like the fan is either binding due a bad bearing or perhaps a critter in there. Just disconnect the wires, blower hoses, water inlet & outlet, and drop it out. Depending on the type of heater (and there were numerous types) it more than likely be bolted in from the engine side through the firewall.
Take as close of look as you can and if necessary go ahead and remove the fan and motor which isn't difficult. Test the motor with 6V or 12V as appropriate. Replacement motors for either voltage are available from the major old truck parts vendors or you can take it to a shop and have it repaired if necessary.
Hope this helps you and let us know how you make out.
Don
