What variety lemon do you have? Can you post a photo?
Could it be that you just haven't left the fruit on the tree long enough? Do you fertilize it regularly?

I have no idea what variety. I will post a pic but they're large, rindy. Just fertilized. The current crop is just turning yellow but previous crops have been on a long time and still come out acidy.

Volkamer???

It does look like volkameriana; but that is most commonly used for rootstock. It is VERY sour. I considered Volk.. as rootstock for my Meyers but it is slower growing, produces less fruits and larger than Macrophylla; and I don't need any of those factors for my Meyers in the tropics. One advantage Volkameriana has is superior resistance to some diseases.

Okay, so nothing to be done about it?

First you need to confirm that it is Volkameriana. You can look at the details here: http://idtools.org/id/citrus/citrusid/factsheet.php?name=Volkameriana

Be sure and check all the details. If it is Volkameriana, it won't be edible.

It does not appear to fit the characteristics of volkameriana. Fruit isn't as wide vs. length, skin is smoother than indicated, also isn't as large overall as pictured. If it is another variety, is there anything to be done about acid content? Not exactly SOUR as much as very heavily acidic.