Your tree doesn't have enough foliage to prune it safely. Pruning it could set it back a good year or 2. It will fill in on itts own. Just watch fro rootstock suckers and remove them as they appear

Pruning that tree is a bad idea. In ground citrus trees are rarely in need of pruning. Left alone they grow into a nice round shape.

I agree it looks to me like a tree that someone has already over pruned. When it comes to citrus ignore all that stone fruit tree advice about vase shape, modified central leader, etc. Let Citrus be Citrus, only prune if you must control an out of control limb that drags on the ground or to reduce size on an overgrown tree. Give it time and it will fill in.

Thanks for the advice everyone! Sounds like pruning may not be a good idea at this point. Other than pruning, is there anything else that can be done to encourage lower branching and new leaves? I'd eventually like to re-shape the tree to keep harvests within arm's reach, especially as it is planted along a slope. I'm willing to set the tree back a couple years for bringing the size down if necessary. Thanks.

Feeding it extra Nitrogen can help vegetative growth, though it can also greatly lower fruit yield

In time your tree will fill in by itself, IF you give it good care. A tree that size should be fertilized 4 times with a NPK formulation. Each application spread out equally during the growing season.