4.5" and 33's here. Drives fine all the way up to 95mph (haven't gone faster than that). But I also replaced the upper and lower ball joints, all bushings, adjustable trackbars, drop pitman, new steering box, steering box brace, etc while we had it torn apart. Other than being higher up it drives pretty close to factory.
If you regear appropriately for the size tires you run, a 4.0 will have all the power it did before the lift/tire upgrade, which ain't much. Haha. Lol.
Do keep in mind, with larger tires, you won't be able to stop as fast (ZJ rotors are tiny). Also bigger tires act like huge gyros when going fast, making it handle progressively more sluggish the faster you go. If a deer runs out in front of you, you'd prolly be better off just to hit the damn thing. Lol.
I usually stay around 65-70mph for that reason. As others have said - just keep it slow and you'll be okay. These ain't sports cars, especially when lifted. :-)
Speaking of gearing, I've been thinking lately. If I was to regear (i got the stock 3.55) I'd like to have the 3.73 like stock in our jeep wrangler. It's not going to be anytime soon, but if I was to re-gear, what would be a good gearing for the 4.0 with 31s?
1998 ZJ 4.0 work in progress/DD
'97 ZJ 4.0|3.5 RC|31" -SOLD
I DD my 5.9 lifted 5.5" with 33's. I think it drives better now than it did stock. I've had mine up to 90-95mph with no problems. I average 13-14mpg and get 200 miles per tank. Good thing I only have to commute 7 miles one way . But like ratmonkey said it is expensive. I've dumped thousands to get where I am now and I've got plans in the works to spend thousands more.
98 5.9 ZJ: 4.5" IRO + 1" spacers w/IRO front adjustable long arms and rear adjustable CAs, IRO OTK steering, KOR steering brace, JKS quick disconnects, IRO belly skid, Bilsteins, 33x12.5x15 Duratracs on MT sidebiters, ARB bumper w/ WARN M8000 and 2 55W Hella Black Magic, JCR stage 2 sliders, Bushwackers, AEM CAI, Kolak Ignition Upgrade, 3" Kolak exhaust w/Flowmaster 44
In the garage: 8.8 w/4.10s and LSD, Artec 8.8 swap kit and truss