If I'm not mistaken, step ups are considered a hip dominant unilateral movement. In that case, you could perform single leg back extensions or single leg romanian deadlifts. But, why not just step up onto a regular flat bench? It's probably not as high as you're looking for but I don't see how that'd affect it too much.
If you're just looking for a unilateral movement then lunges, reverse lunges, pistols, split squats, or single leg squats (all quad dominant unilateral) would be fine.