We have used the bucket on a front-end loader as a support when working on/under my uncle's dump bed. On his dump truck (80's model GM- I don't know specifics) there is a bracket made to insert a post(?) somewhat like a engine hood support. I don't guess we ever had the post, and we weren't ever under the thing for bodywork...
I'd use 2- 6"x6" posts on end as a brace. Find a good attachment point on the truck frame & the bed frame to tie off or bolt through. Stack up a bunch of tires. Or, use a steel gantry crane (minus the casters) assembled in place. I'm just throwing out some ideas- you could even frame a 2"x4" structure under there (think "lean-to") if you think the bed will be raised for quite a while.
Edit- Even if there is a designed/functional/factory way to lock the bed in the upright position, I'd still build something else if I was going to be under it.
Brad
