Also can you shift into 4Low on the fly? Or only 4High on the fly?

 

XJ's aren't designed to shift into 4wd "on the fly". They're the conventional "stop, switch to 4wd, and go". Mine was doing the whole "part time" dash light may come on, it may not for a little while but it's fine now. I don't know what was causing it to not come on, but it's pretty easy to tell if you're in 4wd or not. From what I've heard the 242 isn't as strong as the 231 so you're actually doing better without it.