I don't see why the rear track shock would go bad any faster than the front track shock. And even if it did, I have a hard time believing you'd notice it if the front one wasn't also bad.
If you are buying rebuilt shocks, that's good. But, do you have any idea what valving is in them? My experience is that if you are under 240 lbs on a 121-inch XTRA-12, you don't need any compression valving in the rear shock. And only a light rebound stack is needed. Oftentimes, the stock coil spring on the rear track shock is too stiff, too. For the front track shock, the stock valving, at least on the earlier 12s, was a straight, single stage stack. A stack with two stages works much better up there, since that front track shock seems to do most of the work in the stutter-bumps.
I've changed the rear shock with the suspension in place, and I determined I would never do that again. Removing the suspension entirely is way too easy.
I hope the season lasts longer than you think it will.