I know this is an old post, but figured I'd add what I did in case anyone stumbles upon the post like I had. I drained the rear diff on my Flex and figured I'd go through the procedure above, before realizing I had blocked my welder in. I got to thinking and looking at the rear diff and the flange at the bottom seemed to be the perfect size to drill/tap for a plug. Used a 1/8-27 NPT tap and plug.