My question - is ti OK to rung on/off road tires on the back only of a 2WD Vanagon?  And ditto for pure off road tires?

Years ago, many people ran different snow tires on the rear of their RWD car and often 2WD trucks ran a more aggressive tire on the rear only.
However, I think this was before radial tires, or at least my memory with the snow tires is certainly before radials (and common FWD).

I am wondering about carrying 2 spare tires with more aggressive tread that would allow one to get more dirt road traction.  The idea is running the street tires on a long trip, do the switch over at the destination area, but be able to run them on pavement as well - that is not have to switch them out every time you return to pavement for a day or 2.