8 1/4 ring and pinion from 2000 durango
If that unit is from a Durango, I would almost guarantee it is at least a 3.55, and MAYBE a 3.91.
The posi unit would require custom axles to achieve the 29 spline pattern you need. My car is currently undergoing this mod, and I ordered my axles from Dr. Diff. If you run the diff, you will need to know the diameter of the center pin. There were different versions. Mine in particular was a narrow pin version (with the pin being almost .10 inches narrower than my stock '75 pin), which allows for a thicker, stronger "button" on the end of the axle.
Stock axle versus new axle:
If that unit is from a Durango, I would almost guarantee it is at least a 3.55, and MAYBE a 3.91.
The posi unit would require custom axles to achieve the 29 spline pattern you need. My car is currently undergoing this mod, and I ordered my axles from Dr. Diff. If you run the diff, you will need to know the diameter of the center pin. There were different versions. Mine in particular was a narrow pin version (with the pin being almost .10 inches narrower than my stock '75 pin), which allows for a thicker, stronger "button" on the end of the axle.
Stock axle versus new axle:
