yep its 24mm, and if you cannot weld a nut onto it then i would just whip it off with a chisel mate, bet you anything someones put an inch socket on there once too often and rounded it off.

 

Man mine is rounded, I have used a grinder to put a little slot in the middle of the plug and I bashed the crap out of it with a cold chisel and hammer.. It ended up cutting through the bolt, rather than loosening..

I have tried 6 sided 24mm socket, 12 sided 24mm spanner I especially bought for the pickup.. Next step is welding.

 

x3 or 4 or what ever it is so far on the cold chisel. have gotten 3 out like this so far (2 different trucks)

 

I know this thread is old but I just ran into this problem yesterday. I don't have a torch so I hit it with a hammer. No luck. Finally I just stuck the ratchet on there and positioned it at about 4'oclock. Got my floor jack and raised it right to the end of the ratchet handle. Moved out of the way and gave it 1 pump. Bolt broke loose instantly.