front wheels will not lock in on 1999 Ford F-250 Super Duty

front wheels are not locking in. what should i check first

Asked by for the 1999 Ford F-250 Super Duty
Make sure it is going into 4x4 mode, if you have automatic hubs it is possible one the hubs is not locking. I know some came with auto. or man.
Thanks for all your help.
1 more answer , 1 more comment
The front auto hubs require engine vacuum to operate. Many times the vacuum hoses become brittle and fail at the connection at the front hubs. The hubs should have an "auto" and "lock" position. You could turn them to lock but they can be difficult to turn.
Thanks for your help.