Andy,
It looks to me like the bedside is actually pretty straight, but just twisted a bit and maybe bent slightly where it connects to the rear cross-sill.
I'd try using a come-along and with the tailgate open, pull the top of that bedside inward (anchor on the bottom of the opposite side to keep from bending it. If you can get it slightly inboard of the top of the tailgate, it should rebound a bit. Easy does it till you get it where it needs to be. Maybe loosen the bolts between the cross-sill and bedside first before pulling.
