ok let's check with an image
I really don't understand why one parameter can't be measured if it's said to be a 6DOF and not a 5DOF!?!?!
moreover gyroscopes measure angular velocity around themselves...not influenced by the force of gravity :~
So here goes the image
I made it very simple 
can you please answer the questions at the bottom simply with a Y or N?
in your setup (I'm deducing) can the following be measured???
1.1 Yaw = N?!?! right?
1.2 Roll = Y
1.3 Pitch = Y
1.4 - 1.5 - 1.6 = x y z = Y Y Y
In my setup - can the following being measured??
2.1 - old Yaw - new Pitch = Y? N?
2.2 - Roll = Y
2.3 - old Pitch - new Yaw = Y? N?
2.4 - 2.5 - 2.6 - x y z = Y Y Y
What I really need for my project is (considering the my sensor positioning pictured right)
to be able to measure
x, y, z, ROLL, and PITCH (old yaw, blue line)
Will it work with Sparkfun's 6DOF Ultra-Thin IMU
or will I need one of the 9DOF? ( 9 Degrees of Freedom - Sensor Stick (Old-School) - SEN-10321 - SparkFun Electronics & 9 Degrees of Freedom - Razor IMU - AHRS compatible - SEN-10125 - SparkFun Electronics)
thanks for the patience