so, im a little confused by exactly what the driving characteristics are problem wise. Can you tell me if this is correct, are you saying that in 2nd and 3rd, there is a problem with the smooth application of power?

Also, Im not sure that the backlash that you have measured in the drive train is really a "symptom" as related to power application. At most, you are talking 3.5 inches on the outside of a tire and you are measuring ring and pinion lash in the rear end, and gear lash in the transmission. The tire would be magnifying any movement as you measure the outside point of a large disc, plus movement like that is pretty standard on most cars IMO.

In fact, you've demonstrated it on your other cars, so that really rules it out as the cause I think, UNLESS you can find a worn U-joint or flex discs

From what you have written, could it be a binding throttle linkage problem? a sticky part where you can't give it just the right amount, and either give it too much or too little causing the car to buck? There have been lots of conversions that ended up needing a simplified throttle setup to address that. Maybe its on the throttle end and not in the drivetrain.

Can you describe the way it acts again?

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