2001 Dodge Dakota rear differential

The pinion shaft is part of the pinion gear. Those are real beefy so if it broke on the shaft, I'd suspect a lot of abuse or the bearing went out first and allowed the shaft to get chewed up. That would be compounded by the gear lube leaking out and all the other bearings and gear teeth would have substantial wear. You might be better off just getting the whole axle assembly from a salvage yard. Problems with them are not real common so your chances of getting a bad one are very small. Just be sure to get one with the same gear ratio.

Depending on which axle you have, you may need a housing spreader to get the center section out. Look for a round hole, about 3/4" in diameter, as I recall, on each side of the rear cover plate, and inline with the two axle tubes. That's where the pins hook in for the spreader. That tool spreads the center of the housing to remove the pressure on the carrier bearings, then that assembly can be slid out.

Be aware too that there's a lot of very critical and precise adjustments for pinion depth and ring gear position to set the amount of backlash and tooth contact pattern. There's an art to setting axles up correctly. That's why you're better off with a complete assembly from a salvage yard.