Braden, those modes are of course created by a room about 8X11X25(1130 ft./sec speed of sound divided by twice the dimensions since the sound makes a back and forth circuit), as yours is. That isn't bad, and it can't be said that your room "sucks" as does that of certain of our members who shall remain nameless.
In addition to the room modes, there's the matter of reflections, which all listening rooms other than an anechoic chamber have and which contribute to the sense of spaciousness in the sound from the speakers. Too "live" a room isn't good either, but Dr. Toole in his new book cautions against overdoing room treatments beyond typical room furnishings such as carpeting which help to keep things under control. In particular, he emphasizes the beneficial first reflections and cautions against trying to muffle them. It's possible that the answers to your question may be none and nowhere if your room isn't excessively live at this time.
As to the back surround speakers, they should be on the back wall just high enough to clear the door to the garage and spread as widely as possible, say about 6" from the side walls. If that leaves them too close to the ceiling(say less than 4")then a few inches lower on the side walls about 6" from the back wall.
