Diene, I want to second what OD said. Have you talked to your coaches about your fear? I think if you let them know that you want to get over this fear, and you're working on it, they should be okay with you doing step-ups and not jumps. Just tell them you're working on it, and when you feel ready, you'll jump.
A few months ago at my box we had a hard workout with a lot of box jumps in it. My coach actually had several of us time how long it took to jump v. step and we discovered that (for a lot of "jumps") it was actually faster for most of us to step. Our coaches also suggest that any of us in the over-30 age group step DOWN instead of jumping down.
The ONLY injuries I've ever gotten at Crossfit were from box jumps, both on the same shin. For me, it's a matter of where I'm looking (as a previous poster said). You go where you look!