If you are careful and mark the caps, you can remove the sure grip from the 3.23 and the open diff from the 3.91 and swap ring gears. Get a dial indicator and measure and record the backlash before you begin on each centersection in several spots around the ring gear. Use a punch to mark the caps for position. Don't touch the pinion nut on either chuck!! Remove the caps, remove the differentials, remove the ring gears (bolts on ring gear should be LEFT hand thread!). Reinstall the ring gears after cleaning the mounting surfaces very well. The torque specs for the ring gear and caps are available on line if you hunt a little... Reassemble carefully reusing the same bearing races and caps that you marked. Set backlash identically to the previous measurements, and poof. Instant open 3.23 and Sure Grip 3.91. Should take less than an hour if you have air tools.
