There's arguments supporting both.
Shackle to the rear will track better and generally drive better on the street. This also puts a solid mount at the front that can be easier to slide over obsticles. Down side is the driveshaft length needs to be dead on and it needs lots of slip built in to compensate for the fact that the housing will move forward on downward travel.
Shackle at the front will pull the housing back as it droops, thus compensating for driveshaft slip- this means you can get alot of suspension travel without much driveshaft travel and pinion change. Downside is you (might) have shackles hanging down that get hung up on occasion and generally more foul street manners.
Pick your poison.
