All My Rear Bearings Get Toasted

ok, what am I doing wrong? I have multiple truggies, different brands, but all of them seem to steadily grind up the outer rear bearings in the rear wheel hubs. It seems to me they take a lot thrust back there and I have seen new sets go bad in half a day at the track. I've tried shimming them tight and loose to no avail.
I am not one to let a pesty little problem get the best of me, and am about to make my own rear wheel hubs so I can upsize my bearings. But before I do, I wanted to hear what others have exeperienced and if anyone overcame this issue some other way. Thx all!