Pulls to the right in 4 wheel drive only
 

My '02 F150 Screw started pulling hard to the right in the snow this past winter.
In 2WD it runs straight, only pulls right when in 4WD.
1.Vacuum seem to work fine. Actuator moves in and out about a 1/4" or so. (not sure how much it should move)
2. Raised front off the ground and tires spin in opposite directions. Turned steering wheel sharply both ways and wheels still spin opposing each other.
I've done a lot of searching and reading.
Any trouble shooting help or advice would be much appreciated.
Thanks.

Kinda makes me wonder about that actuator.................

Can the actuator be removed and tested without destroying it?
Does anyone know approximately how much the actuator shaft should move?
Thanks.