UPDATE AFTER TALKING WITH GEICO

After the utterly unhelpful response of Geico customer service, I escalated my inquiry by reaching out to several Geico executives. It took a little back and forth, but today I had a call from a Geico claims supervisor and we had a good conversation about the "rules" of offroad use and my Geico policy. She is supposed to follow the conversation by sending their written policies, but I haven't received those yet.

I was clear that I was just trying to understand the rules so that I could make informed decisions about how to use the vehicle and/or understand when I might not be covered, and that there was no claim to be made on the vehicle.

First off, Geico has no concept of "offroading" -- it is not a term they use.

They generally exclude four types of activity: racing, speed, demolitions, and stunting (stunt activity). It is this last one that applies to what I would call "offroading."

Gravel roads in national parks, BLM land, USFS land, etc -- covered

Mixed gravel/dirt/rock roads, pretty rough, maintained by some agency -- that an ordinary car would not be able to drive, but a vehicle like my 4Runner would -- likely to be covered

Desert wash which is commonly used as a road/trail -- likely to be covered

Desert wash which is commonly used as a road/trail, but with big rocks and I'm driving 50-60 MPH -- not likely to be covered

Flip my 4Runner while rock crawling -- not likely to be covered

Because she is also in San Diego, we specifically talked about some types of trails in Anza-Borrego and areas like the mountains near Ramona. It really came down to being covered unless doing something where there's an obvious hazard to the vehicle, e.g., rock crawling. Desert washes, crappy rocky roads in the mountains are good / likely to be covered so long as you're not doing something stupid.