91 2600i 4x4 not locking in needing help

Ok I have a 91 2600i 4x4 I can't get the 4. Wheel drive to lock in

It has to be a sensor somewhere I put the truck on stands and locked in 4 low and front wheels don't turn I checked the vacuum thing on the front dif it does lock in and out but for some reason this is not locking in anyone got any pointers any and all help would be needed and welcomed