They also rebuild the whiteline flat out group-4's which is basically an AST 4100 series shock. I have 2 sets, and atleast one set is getting sent in ASAP. Its rare to come across rebuild deals, so if you have a set of shocks that can be rebuilt, you may want to consider a way to make it happen. ast-usa.com
For AST's they normally charge $600, so $300(75 a shock) in this case for a basic rebuild. Other shops charge upwards of 2-300 a shock. I was amazed to find out AST was the cheapest when i was sourcing places to rebuild the whitlines...i figured they would be the most expensive. AST outlines what a basic rebuild entails on their site.