alright, I have actually driven a 3.8 JK on 37's with 3.21's. I have a buddy who will spend all his money on things that make his jeep look cool instead of things that make it work. his newest creation is on 40's with 4.10's...
I guess my question is, are 5th and 6th even usable on the highway? the one I drove barely got going in 1st and 85 was accomplished by staying in 4th gear. it will be EXTREMELY noticeable that you have 3.21's, not just a little sluggish. it will barely be driveable. keep in mind that you came from a 5.7 Hemi, a 3.8 JK (or any JK for that matter) is going to be a snail in comparison.
with 37's and a 3.8, you are looking at 5.13's, not 4.10's.
you need to get gears specifically for the JK dana 30 and 44. ours are very different and every website has them specifically labeled "JK" with a subsequent markup because people will pay it.
numbers arent always stamped in the same location, the only place I could 100% say they would be is on the inner face of the pinion which you could not get to.
jack the jeep up, spin the tires, and count the driveshaft rotations. there will be a huge difference between 3.21's and something more appropriate and you dont even have to turn a wrench.