Car goes sideways?

I found a good mechanic when I was looking to get bushings pressed in, most of the machine shops didn't want to touch the A arms. After the front end was done, I took it to the same guy for an alignment. He tried to do a 4 wheel align, but my rear wheels were to deep for the light/laser thingies to attach. He said could be an issue with the rear end not being aligned correctly, not that he thought there was on with my car specifically, just something he learned over the years. I haven't had any sideway issues when my other personality takes over....saving that one in case I ever get a ticket.
My truck, on the other hand, goes sideways all the time in the snow/ice here in the winter and it's got an open rear.