So we're dealing with the differential not locking, rather than not going into 4wd? Have you checked the fluid level? Checked to make sure there's nothing physically blocking the travel of the dash lever, or the control mechanism on the diff? If it's able to lock going downhill, then that condition shouldn't be too hard to duplicate in the garage. Now would be a good time to lube the levers, and the cable, because it eliminates those as a suspect, and it's good maintenance.