does it matter if you mount sleeve bags on the axle, or behind?
no, either way they ride AWESOME!!! at least the 9000 series firestones do
Both will work fine. Either way you go though, you will need shocks to limit the bag so it wont overextend itself.
Here's mine,slightly forwardward of the axle.
To be honest you'll probably get a better ride behind the axle, but a little less lift. I'm sure they'll ride pretty good either way though. Im not good at posting pics but i have a pic of my setup in my profile if it helps any.
you can mount them anywhere on the axle and get the same ride and the same lift, nothing is changed, but they are commonly behind the axle just for a cleaner and lower setup.