I'm going to throw in my experience on bearings cause I too have searched the planet for something that can handle a bit of thrust. At a minimum, it'd be nice to not have a grindy bearing each time I take off one of the wheel hubs. Unfortunately, I haven't found the bearing. I tried Acer ceramics. Fried them right up. In fact, they were shot before my steel bearings went out (I specifically mixed steels with ceramics to compare life having similar conditions). They costed me a boatload also. I ended buying mass quantities of steel bearings and just toss em when they get gritty. The friction loss in ceramics is neglible to me considering the $. I do run ceramics in my Neu motor, however (cause the loading is easy on the bearings).

Now my problem may not be entirely the same as yours. I find the root cause of most my failures is dirt working their way in. I have not found thrust to be an issue in the front/rear diffs. It sounds to me like you had a bad shim job which will toast any bearing. Once they are sloppy, you cannot shim again to compensate. Your bearing will be out of concentricity with the race. Are you certain with new bearings and a good shim job the problem keeps happening? The spiral cut gears require very close tolerances, more than one would think.