How to get my horse less spooky from the ground?

My little arabx pony Shakira is very spooky from the ground. Everything terrifies her, shes really jumpy. When I'm riding her shes much better, (hardly spooky at all) but if I'm just say, leading or grooming her shes very nervous. Of everything. Example: I am riding her past a tractor (the tractor is running). It definately catches her eye for a few seconds, but I just have to shift my weight slightly and I have her attention back.Hardly noticable. 5 minutes later I walk her inhand past it. She leaps into the air with fright, then shivers with nervousness as she walks past beside me. Even once we are past, she is still visibly nervous. When I first got her she was much worse, but as she learned to trust me it has improved heaps, altho she is still very spooky. I've desensitized her totally to umbrellas, tarps, balloons and dogs, but I'm looking for some advice. :)