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Sounds like a 6 foot cheater bar and a 400 lb sumo wrestler is what ya need to get it tight.........
On a more seriosu note, i did use a 4 foot cheater bar to "snug" mine, it hasn't moved since.
Edit: One other thing I have used in the past has been a small piece of bicycle inner tube as a washer where the bumper meets truck for friction creation, holds like glue.
I have had mine off several time due a plow mount and it has never moved and I just tighten the crap out of it. I would not use red loctite because if it moves again or you need to remove it for other work it will be a bear to break the bolts free. I suggest blue loctite and I would take everything off and clean it good, make sure there is no rust scale, dirt, etc.. between the pieces that would keep it form getting good metal to metal contact, out it back together and you should be good.
Thanks for all of the suggestions, I ended up putting on lock washers, and using red locktite. I torqued the bolts to 130 ft lbs.
It didn't move on the way home..... But we'll see what tomorrow brings.
I'm going to switch out this bumper (small dent) with my spare when I get back to Florida, so If todays fix doesn't do the trick , I'll break out the welder.....