Firstly, welcome to the forum. Ottawa eh? Gotta love them Senators at the moment.
Anyway, the axles you have are the 88s. The other ones are shorter and for the buggy and rally models. The fuel you run is fine, you could go up to 25% but I would be wary of 30%. You may need to start messing about with head shims to get the right compression. Personally I would leave the NT16 in there for the minute, it is a surprisingly good and reliable little motor. Remember it is still running in for the first few gallons anyway. There are no aluminum diff cases for the CEN at the moment, if you do decide to cnc some, there will be a few members here who would be interested in getting some aswell. As for your gearing, PoorJudgement is the man to speak to there. He has experimented with pretty much every gearing option there is and then made some of his own up aswell! You could always adjust the changing point to get into 2nd earlier. Good luck with you truck, they are great little things that are more than capable of giving high price opposition a nasty shock on the track.