The pinion shaft seal that is causing the leak is actually pretty easily and inexpensively replaced. One 1 1/8" pinion nut holds the yoke in place that is on top of the pinion shaft seal. Remove the yoke that holds the driveshaft (after unbolting the rear end of the driveshaft) and the pinion seal is underneath. The pinion seal is pried/cut/peeled out and a new one is tapped into place. A 4x4 shop can replace it pretty inexpensively if you're not handy with such things. A seal is only $12-14 and it can be replaced in under an hour.