If you have already replaced the resistor and switch, there are only 2 other issues that I could see causing only High and no Low speeds. This vehicle actually uses seperate power and a relay for the High Speed. It is fed power through a fusible link. The lower speeds are all fed power through the switch and fuse #7 (25Amp). Do you have power to the switch? The other problem could be the relay actually switchs from High Speed power to the lower speed power, it could be stuck in High Speed, but usually that would cause it to blow high with the switch in any position. I would check for power at that fuse and in and out of the switch.
Source: ASE certified Master Mechanic