You could have a steering problem, yet still be in alignment. If the front axle is not stock and has had parts swapped around, the steering arms could be to short or long causing the front tires to fight going around corners. Do you fell this shake in the steering wheel or in the seat of the pants? Theres a term for this, but I can't think of right now. What it comes down to is the inside tire turns to sharp and it drags the outer tire around the corner with it. Once the corner is made, all is right again. If the alignment shop didn't turn the wheels lock to lock, they might of missed it, I'am not even sure there is a check for this.
One more check to make, set the trans in neutral, and one rear tire on the ground, now try and turn the lifted tire. It will be stiff since you are turning the drive shaft if the axle is non-positrack. If you can't turn it at all, it is a posi with bad clutches.
Joe
