'Grabby' brakes

Mark
I can think of at least 3 different scenarios that could have manifested the same symptoms that you experienced; 1)misadjusted rear brakes, 2)incorrect set-up leading to a non-functioning compensating port, and #3) pistons cocking in the bores, driving the leading edges of the pads into the rotors.
As a auto safety inspector for the govt of Mb(ptoui) I have seen many worn-out/under-spec rotors, with nearly worn out pads. And they stopped pretty good. Many owners had no idea. I have personal cars that had under-spec rotors, and worn pads, that stopped just fine.
I have had cars in my bay, where the rear adjusters had not worked for so long that the pistons finally came out far enough to puke fluid all over the place, and the owners had no idea there was a problem, until the pedal dropped.
AS you probably know, rear brakes are self-energizing. I can certainly see rear brakes being grabby if they are doing all the work.Especially if the drums and shoes are so worn and/or out of adjustment that the pushrods end up being dragged around by the self-energizing system.The question then is;why are the rears being asked to do all the work? As long as the C-port is working the M/C will pump fluid out to the front calipers.The M/C doesnt know if the rotors are "barely"undersize. And once the fluid is out there,barring other mechanical problems,That end will work just fine.
What Im suggesting is this; with your symptoms, I draw a different conclusion. Im not speaking to whos right or whos wrong.I wasnt there.I cannot possibly know Im right.