I don't think there's any way to do a diesel conversion without a fair bit of suspension beef up and probably some frame reinforcement too.
A diesel is already offerred in the JK's, everywhere but here in the US, thank's to the ever more intrusive gov't.
I have a Hemi, so maybe a different point of view than some...
Those who haven't had the opportunity to drive a powerful Jeep versus a not so powerful Jeep - could be any pair of similar Jeeps for the comparison - would be suprised at what the extra torque allows. More power = more control and more capability.
You can get the power any number of different ways, new engines, new much, much lower gear, new transfer case with lower gears.... lots of ways.
Sticking with the 3.8, it means 5.38's, 4:1 rock box, rubicrawler doubler or Atlas.... All the gear that will makes the JK shine off road and makes the 3.8 a sufficient, utilitarian engine that gets the job done.
The problem with the 3.8 and the JK's, especially the JKU is that it lacks power on the HIGHWAY and there is just no way to fix that shortcoming, except major expense, like supercharger or engine swap. Older Jeep models sucked so bad on the highway that the fact that, say, a four banger YJ wouldn't hold a 70mph cruise through the mountains didn't matter much, but the JK's are so much better on the road and highway that they need a better engine for that use, imo. The reason I went for the Hemi was for on road driving, which I ultimately do a lot more of than off road driving. The power is great off road to, and don't overlook that fact that any Hemi engine has more torque at 1200rpm than the 3.8 makes at peak, but I couold have gotten the power out of the 3.8 for offroad use through gearing.
The appeal of a plenty powerful diesel to me is range and highway drivability, and like the Hemi, it would also add capability offroad as well. But it needs to fix the 3.8's highway shortcomings.