xflstl is correct. I thought this was much more well known, so sorry about the confusion. When I was talking about the 3.8 auto with 5.38 gearing being "geared" taller than a 3.6 auto with 4.56 gearing, I was referring to overall gearing, which at highway speed includes the overdrive gear ratio of top gear in the transmission. For the overall gearing you take transmission gearing x transfer case gearing x axle gearing. The overall gearing for a 3.6 auto with 4.56 gears is geared lower than the overall gearing of a 3.8 auto with 5.38 gears. That is why the 3.6 auto Jeep with 4.56 gears is turning more RPM at 70 mph than the 3.8 auto Jeep with 5.38 gears is turning at the same speed, if both Jeeps have the same tire size. The difference is in the overdrive ratio of the transmission.
I said "The difference is in the driveshaft RPM, which is way way way higher on the 3.8 with 5.38 gears
you said "The higher the gear ratio, the faster your shaft will turn with every rotation of your TIRES.
We said the same thing. Of course the engine has no bearing on driveshaft speed. But if the engine comes with a different transmission, and the transmission has different gearing, and you use a different axle gear ratio because of the different gearing in the transmission, that will affect drive shaft speed.