The block is a seal driver for the pinion seal. That would be the front seal of the rear end. To repair a leak like that U need to drop the driveshaft and remove the pinion nut and pull off that yoke the pinion nut is holding in place. The other tool is a yoke bar. When U reinstall the yoke that nut holding it in the spline of the pinion gear has to be TIGHT. Real tight. If u just stick a 1in impact up there without using a yoke bar what u r doing is rapping the pinion gear against the ring gear and that aint no good. The yoke bar prevents damage to the ring and pinion when fixing a leaky rear end. (Only very good mechanics will take this step ) If u have ever tried to replace a seal by tapping it with other type implements the chance of frustration exzists due to the way the opposite end u r tapping wants to keep popping out. Any time u r putting a seal a block at lease as big as the entire seal is the ticket 1 good wack and its in and started. This driver needed to be a block because the spline of the pinion sticks out this far past the seat for that seal.Vinnies Farm 2 016.jpgVinnies Farm 2 018.jpgVinnies Farm 2 027.jpg