when I equip my shield guard automatically goes into the first empty slot. . .as far as I know you can't automatically equip it. . .I tried and it put the ability in a slot, but it was greyed out. . .i had to unequip my shield to clear the space and put it back on :\
The way I changed mine is to go into passive mode, then use an /eaction command.
the proper command to do so (in slot 5 in this example, since that's where I keep mine):
/eaction 5 Guard sub
Bear in mind, that I'm not a text command syntax expert, and I sat there trying all sorts of different arrangements of that text until it worked (including substituting "off" for "sub"). If someone wants to correct me, I'm certainly not offended.
I'll try to remember to double check this when servers come back up.