Long arms would be better, but short arms will work as well. Im at 6.5" on my ZJ ive got all 8 adjustable control arms (short arms), rides fine and flexes well for me.
long arms are better. but like he said. they'll work. but it might be a good idea to get drop brackets with that much lift and short arms, i recoment rough country brackets,
i have a 6" of lift, short arms. if you have the cash now, go ahead and get long arms. rides better, flexes better, less wear and tear on joints etc etc.
but if you do get a short arm, atleast get drop brackets or adjustable arms or both.
if you go over 4" get long arms. running anything over that with short arms is just not a good idea no matter who says that they have never had problems with it. you will wear stuff out a lot quicker running short arms on a bigger lift. pony up the money and get long arms, that way you wont waste your money the first time around and if you ever decide to go bigger you will already have the long arms
i'm running rustys short lowers and adj. uppers on 6" lift. If i had the money to upgrade, i'd do it now. The short ones flex fine but are a little rough on the road over bumps.
Get the long arms you will be happier in the end. Get a good set too, there are a few ebay specials or at least thats what they look like similar to these http://www.ironrockoffroad.com/Merchant2/merchant.mvc?Screen=PROD&Store_Code=IROR&Product_Code=IR-X_LAK
Go with a set up from T&T Customs or another reputable company.
I'm currently running short arms with 6" of lift, it rides terrible, does not flex as well as it should and wears out control arm bushings.
long arms are overated. drop brackets and short arms are just as good. the bushings wearing out will last 2 years instead of 3...so what? they are $9. I've got ~8" of lift, rusty's adjustable short arms (upper and lower), and Rough Country drop brackets. rides VERY well on the road, and flexes just as good if not better than most long arm set ups.
Drop brackets...my last XJ had them and needless to say they didn't survive a weekend at Paragon Park. They were the very nice and expensive RE drop brackets.
The upper control arm bushings have been costing me $13 shipped or I could continue getting them from my job(Jeep dealership) at 25 ea.
6" in my book's even pushing it on Drop Brackets... I mean it'd be about worthless offroad w/ 6" on short arms especially without drop brackets... And my body's hurting thinking about that ride..
Here's another question.. Are you planning on running 37/38" tires... If not there's no reason to even run a lift that high...