I am with Escort Radar, manufacturer of the ZR4. From the description you have given of the problem, "laser falsing when engine is above 3K RPM", the ignition system of the vehicle is definitely the culprit.
Falsing of the rear
laser shifter with higher engine revs typically only happens in rear or mid-engine vehicles where the shifter cable is routed too close to the engine. I can't recall any rear or mid-engine Toyotas that exist which are old enough to use points so I'm a little confused why the rear is falsing and not the front modules.
What is probably happening is the rear laser cable is acting like an antenna and receiving the noise coming from the ignition system. Because it is much longer than the front module cable, it is picking up the noise much better. Using a ferrite choke
on the rear laser module cable should help.
Also, you said that you interchanged the parts from another vehicle. Do you mean that you used an entirely different rear laser module and it was still falsing only when engine was above 3K RPMs? If this is true, then did you run this other rear module's cable along the same path as the original?