ATF+4 necessary for transfer case?
 

2000 XJ 4.0L auto, 231 transfer case

I bought a bunch of Valvoline Maxlife ATF for the trans flush and was going to use some of it in the transfer case. But it looks like most people recommend ATF+4. Is there a big difference in the two? and harm running the regular Maxlife atf?

No.
Tcase gets the ATF-3 or DexIII/MERC as your transmission.

Technically the manual calls for Dex III, but Dex III is a conventional base and I prefer to run Synthetic (I stay away from deep water, and I am lazy so I like to max out my maintenance intervals).

I went with ATF +4 (Valvoline full synthetic) the last time I changed my T-Case fluid, and 30K miles later my transfer case hasn't burst into flames or fallen off so I assume it's lubricating just fine.

The transmission is much more picky with fluid than the t-case, and as long as the weight of oil matches fairly closely the transfer case isn't going to complain. I've seen people run Dex/Merc, ATF+4, 10w30 motor oil, 75w90 GL4 gear oil, etc with no ill effects directly tied to the actual fluid used.

Run whatever you prefer - I run ATF+4 and I am probably going to switch to Royal Purple Max ATF when I switch my fluid for the winter (it was only $8/qt vs $7/qt at Autozone last time I checked).

EDIT:
Whatever grade fluid you choose, the manual says the fluid capacity of the 231 is 1.0 liter, or 2.2 pints.

Also, make sure you open the fill port BEFORE you open the drain. If you open the drain and cant get the fill port open, you're gonna have a bad time.

Sounds like I should pick up a couple quarts of ATF +4 then. I've also heard of people running all kinds of things in their transfer cases without issue. "Anything but water" is what I've heard.

Very solid advice, I've learned this the hard way on previous cars.

Don't go thick a sit can't flow through the passages to the bearings. 30w is about the heaviest I would go.

Buy 2 quarts, mine took just over 1.25 quarts IIRC.
You'll only need to buy 1 bottle the next fill which is nice.