honestly i had this exact same problem back when i was shooting the smoky mountains. my 24-70 was great for weddings but it just turned into mud at the edges for high res landscapes. i ended up picking up a used Nikon AF-S NIKKOR 18-35mm f/3.5-4.5G ED for about 350 bucks and it basically saved that trip. its a total sleeper lens because of the variable aperture but the optics are legit. unlike the older d-series primes or even the older tamron 17-35mm non-osd versions the corners on this g-series zoom hold up remarkably well even on higher megapixel sensors. technically speaking the field curvature is way better managed than the old 20mm d-series primes which were designed for film and really show their age in the corners. quick tip for zion... keep it between f/8 and f/11 because diffraction starts hitting those fine details past f/13 on modern dslr bodies. the 18-35g hits its peak resolution right around f/8 across the entire frame and the distortion is way easier to fix in post than the cheaper third party zooms. i found that even at 18mm the vignetting cleans up almost entirely by f/5.6 which is a huge plus for those wide canyon shots. you can usually find these on used sites for under 400 and shipping is usually fast since there are so many copies out there. dont bother with the older af-d version tho because the edge sharpness just isnt there.