For considerably less money consider using the farmers limited slip.  Your
rear brakes are activated by two cables that meet at a junction piece that
ties them into the cable from the brake handle.  In a former life, we did a
little setup that allows one to independently brake the rear wheels.  The
first rig used another complete brake handle with one handle per cable.
Worked fine.  The second rig used a small pair of brake handles in addition
to the first one.  You would set the main handle to almost contact and then
use one of the others to stop the spinning wheel to transfer traction to the
other.  The final design used a motorcycle twist grip mounted on the brake
handle.  The grip mechanism used two gear units from a BMW throttle assembly
and the cables.

Pull up until contact, rotate the handle to apply braking force to the
desired wheel.  The rig went with the 69 Westie when it was sold long ago.

I suspect Scottish Farmers, and other offroaders by necessity and of limited
means have done the same sort of systems.

Great for toodling around Glentanner in the Type 1 LR Pickup looking for
lambchops.  Or perhaps excercising your mussels on some treacherous beach
after too much Lattitude 45 South.

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