First off, When on groomers, I have no problem at all.
Going through powder, I have a lot of pain in the quad of my back leg. I make it about halfway down and have to stop to rest my leg. I've done some research on it and it was suggested that I move my bindings further back. I did that but there is still some pain. Is there something wrong with my technique that I'm missing?
Completely normal. Riding powder can be a lot of work, especially on lower angle slopes. Try to keep your weight centered and only shift your weight back when needed. A powder board would be a good a idea if your riding deep powder.
I have a Mayhem 162 right now. It's not a ton of powder, just off of the groomers off of the trails (if that makes sense). I think I put too much weight on the back because I'm worried about submarining and stopping.
Well, I did just buy a Custom Flying V so that should be better.
I do think most of it is muscle memory the hard way. This is the first season in 3 years that I've been able to go at least once a week. I wish I lived somewhere that daily trips to the hill was very easy.