I'm no expert at citrus but I see you have not had a response.
I looked at your picture and one of the things that stands out to me is the curling leaves.
Here are some links that may help you.
Good luck.
Danny

http://cals.arizona.edu/pubs/crops/az1151.pdf

http://forums.gardenweb.com/forums/load/citrus/msg0602094211826.html

It has a root problem.

Look at the base of the tree. I see a large wound which shouldn't be a problem as long as it isn't getting bigger.

But the bark near the base is off-color. Is it splitting and/or peeling?

Beyond that, the leaves are curling, also off-color, because of the limited root function.

Thanks for the suggestions, I will look into the roots and see what I can find. I am thinking that this might be the problem. When the lemon tree died last year (a few trees to the left) I dug it out and the roots didn't seem right. When the house was built someone ran the drains for the yard right under these trees. Not really a good idea. I'm guessing there may be a lot of stagnant water under the trees.

The bark at the base looks (as far as I can tell from the image) a good example of physical injury from a mower or weedeater. Do you think that might be the case for your tree?

It looks to me as if, for some reason, the tree is not getting water to the leaves. It could be that the sprinkler system is leaking and soaking the roots and rotting them, so that there are no roots to take up water. Either way, I would strip the fruit now, and dig down to the root level to see what the ground is like down about 6-12 inches. I think it is very interesting that another tree nearby has also failed. I would assume there is a common reason. The wound on the trunk at the base also looks pretty bad, but I suspect the issue is below the ground.