manual hubs are a MUST for a BOV..they're not real cheap(last set I bought were Warn, 89 dollars at O'reilly auto in Nashville in '05 for a '90 dodge 1/2 ton). However, if you have manual hubs, and you eliminate any vacuum/electric sliding collar crap your vehicle may have in the front end, you're almost guaranteed 4wd when you need it. I don't really know how a ranger/bronco II is set up, never owned one. My ford has dual I beams with axles that turn any time the front driveshaft is turning, and manual hubs. If the linkage to the xfer case isn't bound or broken, and the hub locking knobs aren't iced over(it happens, might want to keep a can of keyhole-thaw stuff in your vehicle repair kit), then you're virtually guaranteed 4wd when you need it. The cherokee has a solid front end that's engaged to the wheels at all times, the only disconnect point is the xfer case, I can't really improve on that.

P.S: if the bronco does have a 3-axle setup with a sliding collar that locks your front end in(like dodges) or an electric(fullsize) or vacuum(s10) actuator like chevys, you need to know what it will take to bypass that system if you break it in the woods. They make a cable setup for the fullsize chevys...a little looking underneath will tell you if you need to do renovate/re-engineer.