On my 2004 Ford F150 4x4 Fx4...When i turn on my 4x4 and attempt to drive forward..i hear this skipping in the front end and the 4x4 wont engage..When i put the truck in 4x4 and put it in Reverse the 4x4 will engage and work fine..it will work both in forward and reverse...would someone know what the heck is going on..thanks in advance..