What's a good setting for rear shock setting on the 400 EX? I do mostly trail riding....if I find a little jump (and get enough *****) I'll take it.

 

all personal preference. Remember, only adjust your compression OR rebound each time, not both at once. once you find a compression you like (stiffness), then find a set of whoops and play with your rebound. too much and your back end will fling around, too little and you'll load up the shock and have nothing for the 2nd whoop. It takes playing with until it fits your comfort. one persons setting will most likely not fit the next persons setting.
do a search for shock settings and you'll find a post that has a link on how to adjust your shocks.