Check lead from fuse to Hi Lo switch. If you have voltage at the switch and neither Hi or Low work, the switch may be bad.
You might also want to look at the modification at http:/www.bigbroncos.com for improving the voltage to the headlights by directly feeding them from the battery rather than thru the voltage distribution system for the whole truck. It is under technical information.
I did a variation of that proceedure by getting a plow light harness. Details if you want them.
Good luck,

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