Yup, definitely sounds like worms. Actually, the information about diazinon
is incorrect; in the U.S., at least, most of the haphazard wholesale
elimination of chemicals has come to a screeching halt under the new
administration. Diazinon, being an OP, is very toxic and is sure to do in
lots of earthworms if you do a soil drench at the rates prescribed for
grubs. Generally speaking, the site must be on the label, but the pest need
not be (look up your state deparment of agriculture's administrative code if
you're really worried about the legal issues).

I have been told by some old-timers in the business that Sevin works the
best. I have no personal experience with this method, but carbamates are
sure to put worms in a world of hurt, too, so I believe it would work just
fine.

The best approach: make the soil less hospitable for them. Unfortunately,
any approach that reduces the appeal of the soil for them reduces the appeal
of the soil for plants. I'm assuming you aren't looking to create a
wasteland?

As with any pest, the more you learn about its biology, the more solutions
you can come up with yourself to reduce the population. Speculation, but
worth a try: aerating your soil will help microorganisms break down organic
matter more quickly, leaving less for the worms to thrive on. It will also
help when you irrigate. If you irrigate properly (INFREQUENTLY but DEEPLY),
it will drive the worms up and give the local bird population a feast.
Frequent, shallow waterings are unlikely to reach or disturb the worms, and
in addition, will discourage your lawn from developing a healthy, deep root
system.

Me, I'd leave them alone until I wanted to go fishing. ;-)

`chris