the 4matic system shares the same wheel sensors with the abs system. If a sensor is shot both the ABS and 4matic systems will be rendered inop.
if your brakes are 'fighting" against your applying them, the faulty abs/4matic sensor is telling the abs computer that you are slipping in a wheel and is trying to control braking pressure. just push harder on the pedal, the abs will disengage.
The bad news is when the diagnosis is that the problem is on the front driveshaft sensor teeth, not the sensor itself which is in the hub. the whole front shaft needs to be taken out and reconditioned or replaced ($$$) because the teeth are so corroded the sensor gets bad or no info, hence the wonky abs brakes.