probly the rabs unit, for the past 2 years my truck hasnt had rear brakes and i didnt figure it out till a few months ago. my truck used to do the exact same thing youre talking about, slid into the 6x6 pressure treated fence post at about 5mph at my parents house with about a 1/2" of fresh now on the ground and put a nice dent in my bumper last march, with half tread bfg land terrains.......some how the line goin to the rear rusted out at the end of the summer, giving me no brakes, re did the lines from the master all the way back, couldnt bleed the rear, i didnt get it, it was weird, blown rear line the brakes fail, but cant bleed past the rabs unit.......bypassed the sumbitch for the time being till i can get the funding for a new one and rear works damn good. took it out in the snow/rain/ice about an hour ago and it braked like a charm, did a few test 'panic' stops at about 45mph (obviously there were no other cars around and it was on back roads) and she slowed down right quick, slid obviously but not hardly as much as it used to, and the ass end didnt even slide out on me.....also helps i have brand new arctic claws with about 400 miles on them......lol