Having had the same problem, I'd suggest first trying a much higher incline, which changes the angle and effect on the shins. Another alternative if that doesn't work, is to walk very fast, nearly the point of jogging while still walking, again using a high incline. Good cardio but does not create the same impact on the feet.
Disagree on the stepper though-If good-sized steps are used, its very effective if 20-30 minutes at higher intensities is done. Also disagree on the elliptical-if a machine like a Precor that can adjust to considerable incline's used with real intensity for 20-30 minutes, it's excellent. Also feels less like work than most other alternatives, even when the work's still being done, which is why they're popular.