Avenger Superchargers sells a twin screw kit. They're customer service is terrible and they do nothing with the computer. Expect to buy a $600-1000 piggyback ECU like the SplitSecond FTC-1 or Unichip-Q and pay for local dyno tuning if you go that direction.
Hesco sells a roots type kit. They're the leader in 4.0L performance.
RIPP MODS sells a centrifigual kit, with their own piggyback ECU for tuning. Currently seems like the best option.
Either way, expect a viable kit to run $4000-5000, or more.
Not to be one of those annoying guys who responds to a post saying "I want x" with "no you don't, you want y," but if you're looking to drop that kind of cash, you might be better served doing a V8 swap or at least a stroker.
To get someone else to do it? A lot. Probably around 8-10 grand. If you can do it yourself, you can probably get out for as little as 2 grand (maybe) but probably more like 4-5.
I'm not sure how much folks have spent on a stroker. Truthfully, the 4.0 produces plenty of power. What are the other specs on your Jeep? Chances are you are undergeared. Drop $1000 on regearing properly and you'll be shocked at the power you have.
Look into Vortec superchargers. I had one on my 01 Ram with the 360. I then put 4.56 gears and was running 37's and the thing freaking screamed. I was not nice to that truck either, and Had about 40,000 on the charger and it ran like the day I got it. I did alot of research when I started looking into it and Vortec had the best reputation for reliability and quality.