If your fan works on high (and it is blowing hte way you would expect it to on that setting), then cahnces are your fan is fine. I would be more concerned with the switch or the resistor that is between the fan and the switch, this is assuming you do not have some type of automatic temperature control in the vehicle, if you have that then your local ford service department will be your most economical solution.

Edit: I have never worked on the climate control system of a 98 F150, I have worked on the heaters of other vehicles and they are all fairly similar. The fan control cirtuits on a normal non ATC system is actually quit simple.

Edited again: on second thought the truck is probably shot, why don't you bring it to me, and I'll give ya $50 bucks for it (if the tires are in good shape maybe $75).