Thanks for adding so much filler detail from your experience, Megatron! Rich now has a wealth of information. I back up everything you share here.
4.0L EFI is a must, the BBD carburetor on the 4.2L CJs and '87-'90 YJ drives an entire aftermarket in EFI conversions that cost an ample sum. And like you share, there's not that much mystery and no threat in electronic fuel and spark management these days.
You're absolutely right about the 4.0L inline six conversion. At the magazine, I feature the welding and fit involved in converting a 2.5L YJ into a 4.0L. After the Jeep CJ era, AMC/Jeep and Chrysler decided to make unique frames for inline four- and six-cylinder engine applications. On AMC/Jeep CJ's, the V-8s, fours and inline sixes simply required different, bolt-in frame adapters. Actual frames were identical...
Installing an inline six in place of a YJ or TJ Wrangler's four-cylinder engine is no less complex than a V-8 conversion—see the Advance Adapters 'LS' Chevy V-8 into a Wrangler. Four-cylinder YJ models make good candidates for a V-8, although the four-cylinder YJ's AX5 transmission does not meet the torque rating of an AX15. A V-8 into a four-cylinder chassis begs the use of a 4L60E or 700R4 automatic. Advance Adapters is your source for the conversion parts.
Transmission wise, the YJ has the 904/999 Chrysler three-speed automatic without overdrive. The '91-'95 features an AX15 manual transmission behind the 4.0L inline six. It's a proven transmission that I detail in the 209-step, two part how-to rebuild article at the magazine.
Good point about the axle housings for the lower (numerically higher) gears. This is a well taken point for those wanting 4.56 or 4.88 gears in their YJ Wrangler. These larger ring gears will only fit Dana 30 axles designed for OEM 3.73 or 4.10 gear sets and Dana 35 axles with 3.55, 3.73 and 4.11 OEM gears. (Yes, they did use a 4.10 front with a 4.11 rear axle. This is common for many 4x4s and has to do with axle design or, in some cases, the use of two manufacturers. These YJs all use Dana axles.) The YJ Wrangler featured at the magazine's tech how-to was originally a 2.5L TBI with the lower gear ratios that Megatron describes.
Great description of tire needs. The YJ Wrangler project at the magazine is my son-in-law's '87 that I built up—lucky him, eh? We stayed with the 30 front and 35 rear, ARB Air Lockers at each end with Superior Axle shafts for "Super" status; 33" tires, 4.56:1 gears, and he's gone all over Moab and elsewhere.
Think of it this way: Sure, both the Dana 35 and 30 are small, but when you're off-roading, the 231 transfer case delivers 50/50 torque split to the axles. The Dana 35 rear only needs to tolerate 1/2 the torque it gets when the Jeep is on the highway in 2WD high range! So, these axles will work as long as the axle tubes remain straight...An axle truss can help here.
Megatron is right about the NP231, too. Durable for a chain drive transfer case, hardly a weak point! I cover the NP/NV231 transfer case rebuild and SYE kit install at the magazine if you want details on what we're talking about here. A reduction gear set for this transfer case can take low range down a notch for those oversized tires. Then there's the Atlas transfer case—the ultimate transfer case solution. For a stock 4.0L inline six, the NP231, in good condition, will last indefinitely. Parts are readily available for rebuilding.
Megatron is pragmatic and right: The Dana 60 monster axle housings hang so low that any real ground clearance gain requires 40" diameter tires to accomplish! Megatron's 35" tires with a Dana 35 or 30 axle makes perfect sense for useful ground clearance. Strange how these 60s got beneath Wranglers and CJs in the first place. An AMC Model 20 or Dana 44 axle is more than enough. 60s are a lot of unsprung weight mass and very costly to build and adapt.
Thanks, Megatron, you've sparked interest and inspired others to jump into this discussion!