If it's a mechanical issue (shutter problem, etc.), the likelihood of the exact same issue affecting two bodies simultaneously seems incredibly small - it's much more likely something systematic about your setup. Do you see this when you shoot with no strobe lighting - clear blue sky or blank white wall with ambient light?
Something else to try - does the vignette issue change with a lens at f/11 vs. f/2.8?
You see the issue on the LCD, meaning it's in the JPG image (since that's what is shown on the review image). Do you see it when you open the RAW files in your RAW converter (and if so, which RAW converter, and have you tried more than one?).