any one know what i can do? i have a '97 400 explorer and every once in awhile after i back up the left front wheel acts like it is trying to engage. is this a normal thing or is something getting dirty?

 

Try changing your hub oil. it probably has never been changed.

 

While you are at it, jack up the front end and see if the wheel bearings are loose. You should just barely be able to feel it wiggle, if at all. If it seems loose, make sure it is actuall in the bearing, not the steering. Sometimes when the bearing gets loose, the hub can grab the rollers that engage the front end even with the seitch off. Pretty rare, but it has happened at least once that I know of.