I just did this last weekend. I have to disagree and agree with the guy that said it's a waste of time. i bought the stock suspension off a 2013 JK with a towing package. My dad and I spent about 8 and a half hours total to put it on.
First ill go into some problems we ran into.
1. the rear coil springs: The opening on the up side of the JK coil springs are smaller than on a TJ so the don't fit right. To fix this we took on oxy-acetylene torch to it and bent it open more (it needs to be about 2.5 inches wide inside diameter). The Rest went smoothly other than a few hard to remove bolts.
2. the front shocks: how the front shocks mount on the TJ and JK are different. the top of the shocks work fine but the bottom (where the bushings are) are different, i'm sure you can look up a picture to see exactly how they differ. to over come this, we cut open the TJ shock with a sawzaw, where the bushing is held, to get the bushing out, then drilled out the bushing in the JK shock (they say you can press both out but we didn't have a press). then we fit the TJ bushing in the JK shock and bolted it in no problem.
Now here is the real purpose of the agree and disagree, if you are still reading,
I got a little more than a 2 and a half inch lift out of the JK spring.... but only in the rear end of my TJ, the front didn't get lifted at all. I'm still not sure as to why this is but its what happened, but I think it has to do with the JK suspension being a towing package. I have new coils being shipped to me right now that match the 2.5 inches in the rear end so my Jeep isn't slanted forward.
SO IN CONCLUSION:
After about 10 hours of work (assuming it's only going to take a few hours to put in my new springs) and $300 (as the new springs i bought were another $150) I will have a full lift kit (no spacers). so if yours will work all the way (meaning you get a lift front and back) then its TOTALLY worth the little bit of extra work, i mean youre getting a 2.5-3 inch spring lift for ~$150 or cheaper! quadratec sells their 3 inch skyjacker lift for 400. if you run into my problem then you still save a little money anyway.
I know it's long but i think its worth the read.