4x4 issues HELP

My 1993 reg cab b2600i wont lock the front in 4x4. The vac lines are good and electrical is fine. when i put in 4 on the jackstands the front shaft is locked in but i can still freewheel the front wheels. the actuator shaft into the vac pot was seized so i got it moving again and still nothing. tried to lock it in manually and still nothing. Any ideas?