A Pll might do the job, but it would take some electronics knowledge and a lot of fiddling, plus as alec says its not something you want to cobble together for a brake system.
I've messed with vehicle electronics a fair bit but never come across a single abs sensor for the rear wheels, sounds like something bmw would do.
Might be better to go for the 4 wheel system and leave it as unmodded as possible.
The abs system has parameters for the vehciles dynamics such as vehicle sprung and unsprungweight, suspension properties, tyres etc, if you take an abs controller off a diffrent model, even the same bodyshape it may not necessarily work as intended, the car would have to have the same engine, suspension, curb weight, tyres&rims etc.
I found all this out when I converted a classic mini to 2.0 rover power, I was going to fit the rover's abs system, however with this knowledge decided it wasnt a good idea.