My semi educated guess is that the car is tweaked, it is very possible for a BMI car to be tweaked. Check the bolts and nuts holding the rear flex plates on, the ones on the front of the flex plates, in a bad crash they can shift or take a set and tweak the car. Make sure tire sizes are even left to right, both front and rear, make sure you have the same ride height spacers installed the same way in the rear(seen them wrong many times), make sure front ride height is the same left to right I would actually measure the axles from the table with the wheels off.
Make sure the camber and caster are even left to right, both move freely, damper tubes both free to move both directions, etc.
Put the car on a tweak board if possible and see if level, you can actually adjust the tweak on a BMI by putting a very thin washer under one raer link or the other, in between the lower standoff and the link, to adjust tweak if can't fix any other way.
Make sure the diff works well, not tight or loose, and check to see if the body is rubbing on a front tire or hanging up the pod or wires... GOOD LUCK!
Ahhhh, I could'a got a falcon!