I've always wondered that. Looking at my mom's Escape it has the PDU hanging off of the transaxle where the right axle shaft comes out.
Wouldn't the differential in the transaxle handle a different sized tire if it was on the LF tire? At that point, the differential action in the transaxle itself would be able to handle a slight or moderate difference in tire.
Also ... you never drive truly straight down the road. All 4 wheels are usually spinning at different speeds.
A drive system has to be able to manage different wheel speeds or it cannot turn. With open differentials everywhere, this is rather trivial. The challenge comes when we want to have some measure of traction better than what fully open differentials everywhere would provide. This is because if we had a vehicle with three open differentials (front, rear, and center), you'd have an "All wheel drive" system that could get the vehicle stuck when a single wheel lost traction. Pretty terrible for AWD.
Part time 4wd systems are part time because they lack a center differential. Thus, they bind if the front and rear driveshaft attempt to have different speeds. Most 4wd systems not only have a requirement to be used on low-traction surfaces, but also a maximum vehicle speed. (my truck was 55mph, IIRC).
Full-time AWD systems must have a center differential. But to avoid the spinning-one-wheel-and-stuck-with-awd scenario, they cannot have an always- open differential in the center as they (presumably) do on the front and rear.
So most center differentials are either a viscous coupling, a Torsen style, or a clutch pack, or combinations of those elements.
All that to say that center differential operating temperature is the only real issue with mismatched tires. And because the difference in OD across an axle (L/R split)) is halved again (f/r split) at the center diff, the difference at the center diff ends up pretty small, especially compared with the huge disparities possible on snow or ice or other conditions.
