Well, after a long delay over the summer I finally got my Rancho D2 control arms all installed. These are the arms by Rancho with the D2 bushings which are basically the same thing as the Synergy DDB. I did take a NVH reading with my phone app across some railroad tracks this spring with my old arms, but it was so long ago, I don't know if its worth checking again with my new arms. So any semblance of a scientific observation is pretty much out the window. So I guess like every other Yahoo on the forum, all I can do is give you my seat of the pants observation. So here is the process I went through and what I found:
My first order of business was replacing the front upper axle bushings. I had a local shop press out the old Clevite and press in the Synergy bushings that are made for the JK upper axle. I don't know how much work it was but I know they did crush the pasenger side bracket and had to use some heat to straiten it out. I also noticed the bushing now fits slightly loose in the bracket. I was able to jiggile it in and out by hand. I had to be very methodical about it. But I did get it out and back in by hand. I expect that it won't take long for rust to form around it and it will be rusted into place. But it does cause concern and like others I may need to find some adhesive or something to hold it into place better. In hind sight I would maybe purchase the Currie front axle kit and forgo the DDB bushing in this location. Anyway, onto the review of the arms and bushings.
After installing the DDB in the front upper axle mounts I thought I could feel a small of improvement. Maybe placebo effect or confirmation bias. I'm not sure, but it seemed a little better.
I installed the rear arms a few months ago. Again things felt better. A little more planted and less movement in the rear. Again, placebo effect or was it real. Its hard to say.
Finally, after dealing with other house repair projects that took priority over the summer, last weekend I finally got the front control arms installed to complete the whole set of 8 arms. Again I felt improvement. This time it was more pronounced though. The steering on the Jeep feels tighter and the whole jeep feels more solid and planted to the road more firmly. It feels more controlled and less prone to wander. I was expecting this though. So is it confirmation bias or did it really tighten up my steering?
What I wasn't expecting though, was a reduction in harshness through the steering wheel. In the past, driving over rough surfaces always transmitted very abruptly through the steering wheel with an impact and a shudder. Any bump seemed to really shake the wheel. Not bump steer or death wobble or anything like that. It just really felt harsh and loose. The best term I can think of is that the steering wheel just shuddered in my hand. That is gone now. It took me by surprise. I wasn't expecting this. So I still need to drive over some rough potholes and roads for further confirmation. But is seems that the shuddering that I had in the steering is gone. When I hit a pothole or rough patch now, I feel the immediate impact and that's it. Just one impact, not the loose shuddering sensation that I felt before.
So overall, I am very happy with the Rancho D2 control arms. i think they have greatly improved the comfort and control of the Jeep. Now, to be clear, my Jeep had the stock control arms with 110,000 miles on them. Some of the bushings looked really good but a couple were starting to pull away a little. Most were weather checked and cracked. So certainly after 14 years and 110,000 the stock bushings were not performing at optimum levels. I might have gained the same benefits by simply replacing my arms with Moog factory replacements for $200+/-. I also can not speak to how a Currie JJ would have made the Jeep feel.
As far as actual Noise, Vibration, and Harshness. Well, I never had noise and vibration before. So there was nothing to correct. However, as I mentioned above, the harshness/shudder through the steering wheel has been improved. A passenger in the Jeep may not notice the change. But as the driver, there is a noticeable improvement in control and smoothness of response.