just my .02 but as long as u dont try locking it in 4wd while spinning out u cant hurt it. u can shift on the fly at any speed u want again as long as u are not spinng out.
the prolonged use of 4wd will eventulaly cause premature failure of some parts because they were not designed for this type of use. but as long as u dont abuse it it will last a long time.

Scott