If you live where rust happens, they're a poor choice. They tend to crack from wheeling abuse in any climate from what I've heard, but salted winter roads will eat aluminum shims up very, very quickly.
http://www.4crawler.com/4x4/ForSale/Shims.shtml is the site I was going to go with before I switched my plans over to an axle that would require welding new perches on anyways, and thus wouldn't require shims (till I change my setup again... doh!)
Looks like you could be out up to 50 bucks for a set. Still not that bad, since they're actually made from steel instead of being aluminum that will crumble and fall out leaving your axle flopping around like many other brands.
I should note I haven't actually dealt with this guy and don't know if he's good or not.
Why didn't you set the pinion angle correctly when you originally welded them on? It's probably cheaper and stronger to get new perches and weld them on. Or even try to reuse these.
I am
going to add more lift to rear.
I can see the reasoning for not running aluminum shims, but I ran them for at least 2 years and never had a problem. Jumped my XJ and everything. Never cracked or anything.