Im guessing your talking about the transfer case?
if there there is knob u can buy. but you have to crawl under your truck ever time and turn it to get 4wd.
other option is find a manual transfer case out of a ranger or explorer and swap it in.
you have a 4.0 so you should have a 1354E transfer case. you need to find a 1354M. it will bolt right up to your current transmission.
does the tail housing on your transmission have 2 threaded hole on the driver side?
if so the linkage and everything will bolt right up