im not a mechanic, am naive in my knowledge but i will try to help...
the knuckle is the casting that sits on the big ball at the end of your axle (housing)... they have what ford calls trunnion bearings on the top and bottom of the knuckle as pivot points
after '72 ( i think) most design went to what they call a C. Basically two arms holding the front assembly, one upper and one lower. They have ball joints as pivot points.
did you get this car, had it a while, not used 4xx4 and the issue has just surfaced?
or recently had a repair done?
or has it just started happenning, when befor everything was fine?
problem is, the hub that isnt disengaging in 4x4 will be engaging in 4x2 and you may have already blown you housing and diff seals,,,
don't worry tho' - it's an greasy, easy, cheap fix