Thanks for all the advice guys I figure I could give a little back. I've got a 99 B3000 3.0 4x2 that had the same issue high speed only on the A/C and my resistor bank connector corroded away. Well the options are as above you can buy a new resistor bank and plug ~$15 - $40 or if you look at your resistors they actually are perfectly fine only the metal contacts holding them in place as well as the plug are corroded. What I did was patch 4 cables directly to the resistor bank and spliced them into the wiring harness and bypassed the old plug.
It was pretty easy, you tap a hole in the plastic backplate to feed the wires through and just match up the leads to the resistors. Just need to remove some rust off the contacts to make the solder stick but she works like a charm now and it only cost me 4 pieces of wire and $2 bucks for a thermal fuse! Don't make that mistake when soldering, avoid overheating the fuse with your soldering iron =)