Just an update from over the weekend. I tried swapping steppers, no change, tried to loosen the gib, no change. I went to take out the ball screw and noticed that the four bolts that hold the bearing plate were slightly loose. Took the ball screw out and noticed its a bit bent. There are a few bends in it, but more than anything that the machined end isn't essentric with the screw. I bent it a bit straighter, set the preload on the bearings better and reinstalled it. I got the rapid up to 160IPM but after that it would stall pretty consistently in certain parts of the travel. At least I knocked the backlash down from 4.5 thou to 3 thou.

I think I'm going to replace the ball screw, and the stepper with a new 1605 and a 907 or 1200 oz nema 34.

On a side note I hit 420IPM on the X and Y axes. Though the X stalls out if starting from the outside 2 inches of travel. It was pretty cool to see it moving that fast. I think I ended up setting X and Y to 120IPM, 40 Accel, and Z to 85IPM 40 Accel. It will be cool to get them all to 120 Rapid speeds, I guess we will see after a new ball screw and stepper.