Classic symptom of a blown resistor pack, or more likely the fuse resistor in that pack. The resistor unit is in the heater housing, down low in the passenger area. Look for a semicircular plastic dome with wires leading to it. Under that dome is the resistor unit, which simply unscrews. The low fan speeds are obtained by running the current through one or more of those coil resistors, and when they or the little fuse resistor blow, you get only high speed.
Before replacing the resistors, you'll probably have to either replace or relube your fan, since it's possible that the drag and excess current led to the failure, and that squeak was trying to tell you something. The bearings are not readily accessible, but there are ways to fudge it a little and get a little oil in there if you're creative. Otherwise, just replace the fan and the resistor and be done with it. If you do have excess current draw you might not only blow another resistor pack, but melt the blower switch too.
