The rear leaf springs locate the rear so you need something like Ron just showed us, basically you need at minimum two locaters front-to-back and one sideways. Or if you use a four-link with the uppers crossed over the lowers, like Ron's, that'll work. Actually, early '50s Buicks used a torque tube, an enclosed driveshaft connected to the back of the transmission with a spherical joint, and a single Panhard rod to locate the axle sideways.

R.