I need help.

I have a 99 GMC Sonoma 4.3 L ext. cab with the ZR2 package. 4 wheel disc brakes. My problem is the right rear brake continually wears out prematurely. I have replaced the brakes twice in the last 20000 miles. The last time I replaced calipers, steel lines, and all three rear hoses, I also flushed out all the brake fluid and refilled with new. Can anyone help me, I need to replace the brakes again.