My planer tops out at 16", otherwise I'd tell you to bring it over. I could joint it flat in two passes, which would make it flat, but not necessarily parallel to the opposite face, which is what a planer does. You can build or buy a slab jig like the one def and Ox reference, and do an admirable job flattening them. Or you can ask them to do it for you at Sears Trostel up in Fort Collins, or any number of cabinet shops wherever you live. A drum sander is often better than a surface planer for this sort of thing, and most custom cabinet shops will have one. The 'rustic' look with live-edge is in these days, so I doubt you'd have a problem finding someone in your area that could do this pretty easily.