I've "fixed" the steering axle that broke last week by cutting a new metal one out of the 8mm stock I have left over from Tenacity's steel differential shaft using the drill-press-and-hacksaw method to cut the grooves. Even when everything is properly marked and jigged, there's still a good bit of approximation and hand-waving going about. I also took the opportunity to look at the rocker shafts and fix some of the slop in how they fit against the body box. Suddenly, with those re-grooved and the gaps gone, the rover suddenly drives a lot straighter. There is still some drift, but it's over the course of several yards and not just a foot or so.

I also found an issue in the Arbotix Controller code that could see the Dynamixels being allowed to relax vs. being actively driven to maintain their position at all times. This could also cause some unintentional castering as well.  There's one final bit of deflection to be taken care of at the rear, where the back wheel pods are splayed out a bit at the back. I don't think the other Sawppy-'bots out there that I've seen are doing this.  In other words, the system as a whole is working well enough to highlight individual parts that need tuning and general work.
I'm going to be bootstrapping ROS onto the 'bot in the next few weeks, the question is onto what board. The temptation is to install a Jetson or other high-end board onto it, but the real "Curiosity" is really just a data-gathering platform with the processing being done back on Earth, plus space is kind of at a premium in the body-box there, so a Pi is a more likely choice.