Pull the vacuum hoses off and feel each one as someone shifts into and out of 4 x 4 (motor running).. If you have vacuum on both hoses, then the problem is in the slide mechanism or the vacuum cylinder. Tap on the arm and see if it has rusted or "stuck". Might spray some penetrant on the mechanism. Pry with a screw driver. Watch the cylinder and see if it trys to move (hoses installed) when someone shifts in and out of 4 x 4.... last move is to go internal.