15/16" socket
use a 6 point socket to avoid rounding as much as possible, a 24mm socket will work as well, but it isn't as good a fit/a little bit sloppier than the 15/16"
also, make sure you get a thin wall socket, the thick impact type sockets may not fit into the round recess around the plug
@ that time, I would also upgrade to the upgraded drain plug style, it's OEM on later Toyota trucks
http://www.marlincrawler.com/htm/diff/pickup_diff.htm
