Damn that sucks man I have been running mine quite a bit and hard too I haven't had any issues yet my truck is only but a few months old tho too. I packed my diffs with sil glide from napa as strange as the sounds the viscosity of the sil glide seems a little more then stock. Maybe this is helping me idk. But still do let me know how the shims work out for you.
Hows the super cub?