i have an 04 f150, i have been working, trying to get my front wheels to pull,and while jacked up the front wheels will engage and disengage, but when on all four tires and accelerate hard the left front hub will disengage and there is a bad grinding noise. have replaced both actuators and vacuum solenoid, gone through vacuum system and all is good. need help with repairs if someone could point me in the right direction. THANKS FOR ALL INFO AND HELP IN ADVANCE!!!!!