Lots of great information here! Thanks so much, everyone.

Sounds like there's a consensus on the BFG's in the wet.

Some of the more aggressive off-road tires, like the Goodyear Wrangler MT/R and Toyo Open Country M/T, FWIW, seem to come in speed ratings as low as P (93mph). Since speed rating is based on a 10 minute exposure, that feels a little uncomfortably low when cruising for hours across the desert and valleys at speeds near we'll say 80mph. S- or T- rating gives a healthier margin.

The Toyo also has the disadvantage that it's not carried by Tirerack nor, apparently, Discount Tire.

Comparing load ratings via both the numeric and alphabetic code is confusing.

The Firestone Destination A/T seem like a great buy at the moment.

The Goodyear Wrangler Silentarmor A/T seem like quite a decent choice, and also have the M+S (mountain/snowflake) symbol, which means that in California one may not need to mount snow chains.

Well, they ain't decided until she buys them!