Thats some nice machine work and fabbing you did there! I think the 37 spline is a good upgrade for the D80's. That, along with the bigger bearings and 4.10's should make for a great rearend for a tow rig. Now the next question is, when are you doing the 4.10's in the front? Or did you do that at the same time, but didn't mention it since it didn't require any fancy fab work?