Was it leaking from the top or bottom of the shock? If it was from the top I am guessing it was either the shock cap or the bladder (rubber piece). If it was the bottom then I am guessing its a seal or something? Don't know much about shocks. I have the big bore shocks on my rustler and slash. They are good and feel way better from the stock plastic ones.
It might even be the shock shafts if they are scraped up badly.