Which switch? You mentioned three, 4wd, PTO and dif lock.
Wire you found feeding other fuses is supply to this fuse and the other fuses. No need to check from this side off the fuse.
Put a 12 volt light across the fuse that is blowing. No need to have that fuse or a good one in place. The light will provide a load and be ON when a fuses would blow. Do NOT try higher amp fuses, that would be good way to burn up wires or at the very least melt insulation.
With the light in place turn on the switch that was causing the fuse to blow. Light should come on. What ever switch this is, start as far from the switch as possible and disconnect connectors. When the light goes out, what ever is cousing the fuse to blow is down stream from that connector. It could be a shorted solinoid, wire shorted to ground or another wire beside it that goes to a ground. If you do not see apparent damaged wires, most likely the solinoid.