Just thinking out loud here, if you were to work the existing panel, shrinking would be about the last step of the process, it needs to be hammered and dollied/bumped/crowned first before determining if shrinking is needed.
Of course, the double wall construction really makes things a pain, does your truck have a cargo light and interior dome light?
Not the most ideal method, but I have done it with reasonable success on limited access stuff, use the stud welder, weld a stud on a low area, use the slide hammer, not as a slide hammer, but just as a means to grip the stud, pull away from the panel to raise it up, hammer around the stud. Not as ideal as a hammer and dolly on the back, but should help pull the lows up and get the highs down some.