I found elsewhere on the forum the same problem - and the solution. It is the blower motor resistor. Found behind the glovebox it's a pita to get at but doable.
You'll need a 5/16 wrench or socket, possibly a small flat screwdriver (to slip the lock on the connector and remove), a little bit of patience and the new resistor. At Advance Auto it's BWD #RU1045 ($24.99+tx) but they can look it up for you.
All four speeds work fine now. Just have to get that bugger remounted...
Have a look here. Did this on my lunch break.
Heater blower control