Another thing that might help is that the problem (playing in a higher octave) occurs when I go from the lower register to D2 and also starting with D2. I have spent a lot of time on my breathing and can overcome this when I am very focused.
So basically from D2 to G2 something is still funny and you have to adjust your blowing to get those notes out ?
It could also be that the tension on the neck key is not sufficient to make the body octave key engage properly. Once again, the neck - being unoriginal - may not have the correct spring weight to counterbalance the body pip mechanism, thus the body pip is not opening the way it should on D2-G2.
Nicest thing is to always have an original neck for that model (actually the fact as to whether the neck is original to the particular horn is really a lotta hooey). So if you can get a 156 neck, you'd be set (this might require some tenon adjustment by your tech - a 5 minute endeavor of a repair).
Short of that, you could try to find a Bundy or Bundy II neck, as I believe these fit Aristocrats. But getting the 156 style would be preferable.