ok, your speed lines up with mine for your revised numbers. Doesn't the motor feel wound-out doing 72 mph at 2750 rpm? I'm getting ready to regear, and I was thinking of 4.10s for 31.5" tires because sustained 2700 rpm with this motor feels pretty busy to me.
BTW, you've done some very cool mods to your XJ. Are the unibody rails fully plated? Any cracks occurring in the usual suspect locations?
Wound up?
Cruising at 2700-ish RPM , the 4.0L is operating just below the peak output of it's power range.
https://www.automobile-catalog.com/curve/2001/1318895/jeep_cherokee_sport_4x2_2-door_4_0l.html#gsc.tab=0
Living at 4800ft elevation and frequently travel/recreate between 3500ft and 13,000ft elevation, I have zero regrets about running 4:88 gears.
I ran 4:56 gears with 31x10.50 and 32x11.50 tires on my 89 XJ for a few years and this combo performed well.
When it comes to regearing, there are so many variables that never appear on a chart and folks who solely follow a chart are more often disappointed with their gear choice.
Some of these unwritten variables include:
Altitude where you operate your rig (low elevation<sea level to 2,500ft> denser air=engine builds more power and can tolerate a taller gear. 3,000ft and above, as the air thins and HP decreases, your rig can benefit from lower gearing)
Automatic or 5speed manual. Some charts factor for this and the final OD ratio, but using a too tall gear ratio will leave your automatic transmission constantly hunting between 3rd gear and OD cruising. This also builds unwanted heat in the transmission/cooling system. This will also have a negative impact on fuel economy.
Added wind resistance due to lift and increased rolling resistance/contact patch from larger tires/wheels. Roof racks and light bars also increase drag.
Added weight of aftermarket bumpers, winch, armor, full size spare, trail recovery gear, tools, camping gear, bikes, etc.
Power loss due to aging engine
Pulling trailer/ATV's
All of these power robbers should be factored to your situation, which a chart will never be able to consider.
Yes, my unirails are plated nose to tail, this is one of the first mods that I did before I lifted my rig and I highly recommend it. Steering box area is plated on both sides, with welded sleeves in the unirail, so I've never once had a steering box bolt loosen/break. Zero cracking on the unirail in 15ys and 130k+ miles of use, the doors all fit tight still and the unibody doesn't pop and squeak like my non-plated 89 XJ did.