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Just to help future renters I thought I'd give my experience how it went. When we arrived at SFO; there were all kinds of Suv's available except any Blazers of course. We could pick any Suv we wanted and I think we might as well could have reserved just a cheaper intermediate Suv instead of a standard one. We picked a Ford Escape and since there was no user manual in it and there was no sign of 4wd I asked the rental agency guy if it really was a 4wd. He didn't know but called someone who said it was. Later on I realized it said 2x4 on the key ring so it turned out they didn't know what they were talking about.

On the Sierra foothills we ran into trouble because chains or 4wd with winter tyres were required. (Locals told us that 4wd would have been enough for the highway patrol.) Also you are not allowed to put snow chains on a rental. So strictly speaking I guess you won't be able to use a rental in snowy conditions at all!
Read the T&C more carefully: you are responsible for damage caused by chains. They are not forbidden (at least with National).

Btw. They do sell snow chains at places. At the Yosemite gate they asked 60$ for chains, 30$ to put them on and 10$ to take them off.