Even on an open diff both driveshafts can spin the same way, it's no proof that it's an lsd. The way to check whether it's an lsd (and also the condition of the lsd) is to try to turn the driveshafts opposite ways. If it's an open diff it will be very easy, if it's an lsd it will be difficult (or still relatively easy if the lsd is worn out).

Regarding the rest of the rear beam, the 3dr had the arb pickup points outside the spring area. The 2wd sapph had them fixed to the brake pipe bracket on the inner side of the arm. Also something to check is whether the ball joints connecting the arms to the subframe are not worn. Any play will be too much! New ones are about 80 pounds each, so you wouldn't want to replace all 4. (And replacing them by poly bushes is not the best idea)