If you JB weld it, you are effectively sealing the space there. Which means that every time the shaft tries to slide out it will create a vacuum and every time the shaft tries to slide in, it will generate pressure inside that cavity. The BEST outcome of that is that it blows the JB weld right out the end and you get your leak coming back. The worst case is that the shaft doesn't want to slide and it pushes in on the output shaft of the trans and in on the pinion of the rear diff. That's going to be extra thrust load and premature wear on U-joints and thrust washers in both the transmission and rear end. Its just as much work(or actually probably less) to just pull the rear driveline, take the 4 bolts off for the rear trans housing, replace the output seal on the rear housing(because you are already there) and pop a new o-ring in the output shaft. It's probably a 1 hour job... tops. Then you will be right back to new, non-leaking, operation for about $20 and 1 hour of your time.