automatic 4WD

I have a 1995 Ford Ranger with the automatic 4WD which has the selector
switch on the dash to choose 2 WD, 4WD High and 4WD low. To disengage
the 4WD you are supposed change the selector and then back up in a
straight line for 10 feet. I always back up way more than the
recommended 10 feet because you can't always hear when it disengages.

A couple of weeks ago after I had been out on some back roads I did my
back up but after I started forward again I got this awful banging noise
like metal beating against metal. I immediately stopped, did my backing
up again and when I started forward again all seemed okay.

Since I'm assuming that for some reason the 4 x 4 never disengaged the
first time, my questions are:

Why did that happen and did I do any damage to my pickup? How do I
prevent this from happening again? Am I not 4 wheeling enough to keep
everything lubricated or something? I'm hoping one of you guys can
answer my questions so I don't have to ask any of the guys I know since
they probably don't think a woman needs to own a 4 x 4 in the first
place unless she has more knowledge than I obviously have.

Thanks.

Anne