Your rear motor has the tach so when it is slowed down by your finger it is sensed and the voltage is increased to both motors. That is why the front speeds up. You can unplug the 5 wire motor connector and measure resistance on the motors and try running individually on DC with an amp meter in line.
Or, swap the front motors of the bad engine to the good and vice versa to see if the symptom follows the front motor.
If not the motors, possibly some gear lash issues in the truck block causing the stuttering. These problems are some of the hardest to trace down. At least you have a duplicate engine to compare against. G
