Ok, I posted on the BugGuide.net forum and got a pretty quick response
Derobrachus geminatus.
These are indeed specimens of the palo verde root borer. Females are heavy-bodied with slender antennae. Males are more svelt, with thick antennae (at the base at least). They fly clumsily, so biggest threat is of them colliding with you. They are attracted to lights at night as well, so consider shutting off your outdoor lighting. They can bite if molested, so don't pick them up. Otherwise, what is the issue? They don't even cause major damage to the trees they bore into.