I assume you have your stock top plates and isolators? The stock spring isolators should be fine. If you're not so concerned about being the highest you possibly can, go back to stock top plates, iso's, new springs and struts like everyone has said. That's all that should be in the compressed assembly. Do not forget: you'll need an alignment after making this change.
Now, since that should probably get you into the vicinity of 21 inches, as some have mentioned do a clevis lift and a top plate spacer lift. Like peole have said, adding 1/2" to your front suspension will yield aproximately an inch of lift. Since your springs are already broken in, once you drive around the block a couple times and hit a few pumps, new lift settings should remain relatively constant. So you should be able to dial it into a certain height and it will pretty much stay there. So if you're not overly concerned with getting to the exact highest point you can , set it up to be about 22-1/4". Start with a 3/8" top plate spacer and 1/4" of clevis lift (2 conduit nuts?). You may want to add or remove a conduit nut down the road if your setup does not exactly hit the height you want. Weight and such could make heights vary slightly.
I'm assuming you're quite familiar with the clevis lift, but to reiterate, it involves leaving a little bit of space between the top of the clevis and the bottom of the strut, typically 1/4" or 3/8". Most people slide something onto the strut prior to mounting the clevis, such as a stack of two or three conduit (or some peolpe, more) nuts. This forces the clevis to not slip back up the strut.
As for the top plate spacer / shim, people make or buy a contoured plate with holes in it, that slip over the bolts on the top plate. This goes on after the spring assembly is complete, so it goes on right when you're going to bolt the assembly into place and just spaces the spring assembly away from the mounts on the jeep. The picture below is an example. You need one pair: one spacer per side.

If you don't want to make your own, there are places to buy them online, and I have a stack of them in my garage which I sell too. Check the link in my signature for more info.
Sorry to anyone if my info isn't exactly correct. I just tried to make what has already been said a little more clear. It might have been a little confusing to piece together all of the various answers.
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2005 KJ Renegade
Fabrications for sale:
Aluminum Roof Rack (See Page 9)Aluminum Gas Tank Skid --
Radiator Skid --
Front Strut ShimsComing Soon: Rear Bumpers with lots of options