Rear End & Gear I.D.

Have to use the matched gearsets.

So no you can't use a different ratio pinion, 3.21, with a 3.91 ring gear.

Reason being that as the ratio numbers get larger, the pinion diameter gets smaller. The ring gear is thicker to move the tooth over to engage with the smaller pinion.

If you get a chance look at a 3.23 gearset in an 8.75 vs say a 4.10 or 4.56. You'll notice the ring gear thickness.