A relative just bought this truck used, took to shop they looked over were like this things is in good shape, saw nothing wrong with it. He decided to buy it, today went to plow snow found out the 4WD doesn’t work. He said he can just put rock in the back to weight it down instead. To me it seems it is common issue, and would think could be like low fluid of some sort, or maybe some sort of adjustment needs calibration. I don’t know if the enrage for 4WD uses a clutch of some sort and that maybe fails. Or some gear or shaft maybe broke. I would think issue is in transmission area, but could be in the differential in front maybe too.
I tried googling it to see if could come up with anything that may be a simple fix, but finding hardly anything. Almost everything says check fuses, etc which for a truck this age seems kind of silly it would have electronics for the 4WD.