my heater/ac fan works in low on every switch setting except on the highest when it actually changes to the highest speed. could this just be a faulty switch or something worse?

Could be the switch (I would try spraying it with electrical contact cleaner) or the blower motor could be bad. For the different fan speeds the motor engages different phases for the speed.

i've had similar problems with my old '98 ranger let me explain what i found and maybe it will help you out.
my fan for my heat and ac was blowing on the high setting only. if you look under the hood on the firewall were the heater fan is located you will see a block that has 4 wires in it. i pulled that off and found it all corroded up. what i ended up doing is to make myself a little pic of were the wires went and cut the plug off and crimped new female spade connectors on the ends of the wires. while i was in there i decided that i'd take out the 2 screws that held the heater resistor out and found even more green corrosion.

i went to the stealership {because that's the only frekin place that stocked it in the middle of january} and got a new one at the sum of $27 something, then using my little pic i drew up put my wires back on and i was good to go.

hopefully this gives you a good place to start and i didn't confuse you in the process.
maybe this link will explain it a little better, compliments of trs...http://www.therangerstation.com/Maga..._diagnosis.htm