2008 Scion xB - Brake, VSC & ABS warning lights all coming on together and then going off intermittently while driving - dealer says the problem is bad rear wheel speed sensors and that to fix need to replace along with hubs - I can't find where the car even has rear wheel speed sensors

I have the same problem as you, with the same car. It seems to be moisture related. Mine is two months out of warranty, so I inspected the rear wheel speed sensors, like your dealer said. There is a TSB (technical service bulletin) concerning this problem, so Toyota/Scion do indeed know that these sensors are failing. In my case, one of the sensor connectors was broken when the car was assembled, which bent the tiny sensor pin, which broke off while I was attempting to get the connector plugged back into it. I make my living as an electronic technician so I'm not inexperienced with such things. Anyway, rumor has it you're looking at over $700 at the dealer to replace one sensor, since you cannot replace only the sensor. The part (rear hub assy) can be obtained online for $290, plus the wire for $37. All in all, it's a crappy situation. A known problem concerning key safety systems on the car, and Toyota neither issues a recall nor do they extend the warranty.

Hmm... is moisture or heat/temp related? I had both intermittent and constant light issues, coupled with grinding sound in rear. Firestone replaced both rear wheel bearings (which include the sensors), covered by ICW extended warranty (thank G-d), and constant lights went out. They could not verify the cause of the intermittent error code, and has now come back twice, but only after driving about 30+ highway miles. Seems moisture may be a factor... certainly some minor snow today, but wondering if heat is also a factor. Yes, one of several issues with the car. Premature brake part wear/failure (rear pads and rotors every 20K or so), fronts at 60K, rear caliper at 70 (ICW covered it too, no hassle), tires at 30K. Not quite what I had hoped for. FYI... when extended warrantys cover such things with no hassle, it is apparently because they have flagged it as a "known defect" with the car, and they know they need to cover it. Good for them, BAD SCION!!!

My '08 XB has had the same problem intermittently for years. It happened about two years ago, after driving in the snow destroyed the wheel well linings and mud flaps. But after driving in the rain, the lights went off and all was well. Just had some snow here in Hudson valley and lights came on three days ago. Just when out on their own.