I need a front third member for my 87 Zook. I will be rebuilding it with lower gears and new bearings so it doesn't have to be in perfect shape, as long as the spider gears and spider gear case are in good shape that would be fine. Send me a Private if you might have something.
The only way I know of to tell the difference between the front and back is that the front spider gears have a different number of splines than the back, so the axle shafts splines are different in the front than the back. If you have a rear or front axle shaft you could see if one of them fit into your chunk. Otherwise I dont know how to tell just by looking at them.