OK, now I have a decent theory for what's happening.
In the top gear in manual trans or the OD gear in 42RLEs where the torque converter locks up, we have additional vibrations on the driveshaft (I will look into the why of this shortly). The damper acts to absorb this extra vibration at that combo of speed+rpm ranges (which doesn't happen in the penultimate gear), without which we get resonant harmonic vibrations in the TC skid/frame/whatever due to the design choices Jeep made. The 32RH jeeps did not have a overdrive gear, so they did not need a damper. All manuals therefore got the damper with the 4.0L engine. Perhaps the vibrations were not bad enough to induce resonant harmonics in the 2.5L engine Jeeps with any transmission, so Jeep did not put a damper on those. The 42RLE with this stupid 0.69 ratio is really prone to this on 4.0L jeeps.
A good test for my jeep would be to drive it with OD turned off and see what happens. I am going to call Dave kishpaugh first thing on monday to test this. If the vibrations reduce a lot we know we are on the right track to solving this problem (which is having a correctly weighted damper).
how's this theory sound
@Chris @Ranger_b0b ?