Spinning had tightened up my thighs and really brought out the musculature in my calves.
It will work your hamstrings hard if you focus on the back stroke of pedaling (pulling up from the bottom of the pedal stroke). I think that's where a lot of my thigh toning has been - in the hamstrings.
Your lower back should not be bothered by Spinning. Be sure your bike is set up properly; both the seat height and the saddle to handle bar position.
I guess I don't know what you mean by back stroke. Do you mean back pedaling?
Probably TMI but, I'm sore from the seat. Ouch!