E brake issue
Had similar symtoms, shudder when braking, replaced front rotors and helped a bit but shudder was still present so I researched and found the e brake acts up due to rust. After pulling the rear discs off, found one side e brake shoe had a lot of rust on it and was not in the normal closed position, and the mechanism was rusted slightly as well. Cleaned the mechanism, and put anti-seize on the moving surfaces of the E brake mechanism, cycled the E brake 3-4 times to get the anti seize to coat the surfaces, re-assembled, and brakes are smooth as butter again. In my case I surmised that the E brake sticking was enough to either cause the shudder directly, and / or it may have been introducing enough resistance to possibly affect the ABS control due to uneven stopping of the wheels. Not sure, but if rotors have been replaced, turned and / or ruled out as the cause, strongly recomend taking a peek at the E brake. E brakes on rear wheels only. Hope this helps.