You have a blown air shock which costs $1000 + each to replace. I replaced all mine with springs which is cheaper but the ride is not nearly as good and of course not adjustable anymore. I kept the 3 good shocks though so if you want to purchase them on EBay:
http://cgi.ebay.com/ebaymotors/Lexus-LS400...039757939QQrdZ1
Mention you are a member of this club and I'll give you 10% off
Cheers,
Colin :) :)