No dyno test here in my area, all that is done is the tech runs the engine up to 2500 rpms, holds it for X amount of time, then releases the throttle, after it drops down to idle, if the machine gets a good reading test is over, if not they redo the test.
From taking a look at Rockauto, it looks like the 2.4 trucks, Camry, Cressida, and Previa CA smog equipped vehicles got the second O2 sensor for the '91 model year, with other models getting them over the next few years.
If the rear O2 is just a nanny for the Cat, then a leak there wouldn't effect the mixture, which kind of puts it back to a cold cat. Given how fast this test was done compared to those in the past, or possible a combo of a cold cat and aged sensor, might poke around and see what the sensors are reading ohm wise.
