It could be a difference in the way your motors are timed. Timing in regards to a brushed motor refers to it's bias. There are forward timed motors (faster in forward than reverse). There are reverse timed motors (faster in reverse than forward). Then there are neutrally timed motors (same speed forward and reverse). These tanks are supposed to have neutrally timed motors, but I'm sure there is a +/- calibration spec in the factory where the motors are produced. That calibration deviation is probably what is causing it in this case as you have already tried all the other things I would try (track tension, track alignment, or any binding in the gearbox/drivetrain). One motor runs forward and one in reverse to make the tank go forward because the gearboxes are built as a mirror of each other. I don't thing there is any easy way to retime these motors without completely disassembling them. I wouldn't recommend that anyway. My guess would be that both motors are slightly forward timed which makes the difference between running one forward and one in reverse greater causing the noticeable delay in starting the motors causing the yaw. You could disassemble each gearbox and swap the motors around, but that could make it worse. Other option is to get a different set of motors. Once again, that might make it worse if you get a set of motors that even further off. If it's not the motors, it could be a bad bearing in the gearbox or it could be a gearbox that is not aligned right or I've had gears from two different shafts in the gearbox touching each other causing drag. In that case I used a washer to create a spacer fixing the issue.