Hi I have a 2003 F150 XLT 5.4L. It is a 4x4. When i shift into 4L or 4H it will engage and I can feel the truck jolt hard in either reverse or drive. It seems though when I actually go to get unstuck from a spot only the 2 rear wheels spin. I don't understand how I can feel all four wheels pulling forward for a second then I don't. Please help. Thank you.