Yes, there is a proportioning valve. It is integral to the valve block mounted below and forward of the master cylinder. From there is regulates the pressure to the single rear brake line (splits again at the axle), so it can't affect one rear caliper without doing the same to the other.
Are you absolutely sure it was a rear brake? It is common for the right front to lock since the battery and driver are loading the left side. The rear locking means you had the clutch in? To be sure you might look at the the tire. A vibration is easily felt from a front, less easily on a rear.
Does the parking brake work normally? I have seen the parking brake lever arm shaft (where it passes into the casting) get rusty and return spring is not strong enough to disengage the parking brake. You should be able to move the brake lever by pushing it with your palm. Maybe it's dragging a little.
You don't have to spend big bucks to get over 1.0 G braking and keep your 17" wheels.
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