It's not the "by the book" way of adjusting mechanical disc brakes, but I've had good luck just moving the pads until they just start dragging, then backing off a click or two for both the fixed and movable side. If the discs are free normally, but rub when the wheel is loaded, then you might back off another click - I bet you still have plenty of brakes. If loading the wheel is a continuing rub issue even with the brakes properly adjusted, then I'd start looking at bad wheel bearings, binding cables, etc.

- Mark