The DC 2.4L is a completely different engine then the AMC 2.5L. The AMC 2.5L was first designed to replace the GM Pontiac Iron Duke 2.5L that the CJ's had been using for years. In 1984, the first Jeeps with the AMC 2.5L. This included the CJ and the new Cherokee (XJ) and Comanche (MJ). The engine was designed from the 4.2L AMC already had but with some differences. The engine still uses push rods like the 4.2L. Stock gearing was 4.10/4.11 for the manuals and 3.73:1 for the automatics generally but when parts ran short, some 3.54 gears were used for the auto's and possibly 3.73 gears were used on the manuals. The engine has had different ratings over the years but the TJ's has 120hp (down 3hp from the YJ) and 140lbs-ft of torque. The engine hits peak torque between 2600-3200RPM. This is pretty low and is in the middle of the power band.
The 2.4L was designed to be used on multiple platforms. The Neon, Caliber, Liberty, PT Cruiser, etc... Unlike the 2.5L the 2.4L was designed to be a transverse engine instead of the conventional N-S setup a RWD uses. The 2.4L also moved away from push rods in favor of OHC. OHC involve less parts so there is less power wasted in moving the valves. Typically, this also means that the engine is more balanced and can rev higher. The down side is there there is less torque down low. To hit peak RPM with a 2.4L you will need to rev to 6000. That's 3000RPM more then the 2.5L. The 2.4L does produce 147hp which is 27hp more then the 2.5L though.
The first 2.4L's came out with the same gearing as the 2.5L, 3.73 and 4.11's. They were still using a 5-speed manual and a 4-speed automatic 42RLE. Then they switched from the 5-speed manual and 4-speed auto to the 6-speed box for both. They also regeared the 2.4L to 4.56:1 in both models. This only lasted a year, I believe. Then they went back to 3.73 and 4.11's which is where it started till they phased out the 2.4L Wrangler.
As for the 3.8L, this engine was derived from the current 3.8L DC has been using in their Caravan and other cars. The engine was revamped though. A friend of mine bought a Rubicon Unlimited JK and drove it from MI to CA with the 3.8L. He said he likes the engine better then his old 4.0L but not as nice as his 5.7L Hemi. (His 97 TJ ahd a 4.0L before he swapped in a 5.9L. His Unlimited had a 4.0L but was swapped out at AEV for a 5.7L Hemi.)
That said, another friend said had owned a 2.5L, and two 2.4L's one with 4.56's. He never got very good gas mileage compared to his 2.5L. His Jeeps were all relitively stock.
The biggest advantage of having an engine shared with a bunch of other model vehicles is that many parts are interchangable. This means there is a higher likelyhood of performance parts especially with the Neon. There is also the option of a turbo, lower compression rods, etc...
