I square up my rear with a Cheapo Depot laser level. Just index to the metal rim or to the brake hats, axle ends or whatever as long as you flip the level over from side to side. Drop some points from the frame or body to develop a centerline for the chassis and then measure out to each side with a tape from the center by shooting the laser to the tape.

PS, Start by finding a chassis centerline then center the rear housing and then align it to the tapes up front.

I realize that this isn't the same measurement accuracy as factory but better than factory is good, isn't it?