Thanks, did not think about ABS. I now see why GM did not go with the time tested, ubiquitous for a long time, duo servo brakes.
There were GM trucks with ABS & Duo Servo drums on the rear, Just about every GMT400 2500 & 3500 in both RWAL & 4WAL configurations depending on the year. GMT400 1500's even had a optional heavy duty 11" Duo Servo drum upgrade over the 10" Non Servo drums that were standard.