Nice MX shots! I first used this lens doing that same work, but with a D100. Question is, what aperture are you using? I used to shoot between f4 and f5.6 when shooting MX. My issues are wide open at f2.8. I will attach an unedited jpg or two to show you what I mean.
I usually keep a skylight 1B filter on there. The vignetting is very noticable throughout the zoom range with the filter on; but I see it with or without the filter on the long end.