1. Verify wheelbase side to side
2. Put car on an accurate 4-wheel alignment system (Hunter laser system would be good) and get a rear thrust angle reading--if it is out of specs, that is reason enough to chase this problem.
This should give you some indication of the degree of problem.
As far as the TSB, the problem was advertised as improper "piercing" of the front lower control arm pivot hole--supposedly on the left side only, as I recall. Since it is very difficult to access the area of the frame that would need to be "fixed", the solution that GM offered by the TSB was to elongate the hole in the lower control arm bracket on the rear axle to allow the axle to be repositioned to "match up" with the other side of the car--essentially "swinging" the entire axle rearward on the affected (left?) side. I also recall that the TSB makes no mention of checking thrust or pinion angle, but those would need to be checked & rechecked if this process is carried out. The bulletin also does not direct that a doubler or locating bushing/washer be added to hold the axle to LCA in its new position, but if it is done following the TSB, I would only do it in this way (by adding something to positively fix the mounting position).
The pinion angle question was addressed in part by upper control arms that were 5mm shorter or longer, available from GM--again, no mention of these in the TSB.
The interesting thing about this TSB was that it was supposedly issued to address customer complaints about the visual difference in wheel position (ie. cosmetics), not to correct any technical problem. I just wonder how much better the car might drive if it was properly squared-up?