Thanks, this is what I was try to say in my earlier post. Your thorough explanation and graphics make it clear.I want to share a simple diagram I made that illustrates the suspension bind myself and some others were talking about. Even if its not the root cause in this case, it is worth considering when looking into suspension components with compliant joints vs rigid joints. (on OEM suspension geometry)
This is not too scale, but representative of the side view (what matters here) of the Bronco rear suspension (as well as most OEM 5 link set ups).
The upper control arm and lower control arm are different lengths and not parallel. The angle between them is more relevant than the difference in length in this case. Because the arms are not parallel, (and to a lesser degree, different lengths), as the suspension moves up or down, the axle rotates forward and backward. The primary goal of this is to help counteract the forces that squat or jack the rear suspension during forward acceleration or braking. You can visualize this by imagining the torque induced at the axle by brake or driveline torque, and how it wants to lift or lower the suspension. For example, you step on the gas, the wheel torque is counter clockwise, the reaction torque at the axle is clockwise, which wants to pull the suspension down because of the angle of the links. This helps counteract the rear squat that naturally occurs as the vehicle accelerates forward and weight is transferred to the rear. The opposite is true for breaking. A secondary goal of this is to help the axle pinion angle stay more aligned to the transmission or carrier bearing throughout the suspension travel. And of course beyond both of these goals there are all kinds of packaging limitations, costs, weight, durability, and NVH to consider.
Now lets look at the suspension when one side is all the way up, and the other is all the way down. (again, not to scale)
You can see that if the joints both at the frame and axle have no compliance, the axle is forced to be at a different angle between the left and right side. Axles are very strong, but when you pit it against the frame, and very robust linkages/joints, and brackets, its going to slip. If you make the axle stronger, one of those other things will give out or flex (flex may be fine but could eventually lead to fatigue failure). From the OEM perspective, the bushings themselves give. This not only helps NVH, and long term durability of the joint, it allows the axle to behave and handle well on flat surfaces as well as give the compliance needed to articulate reasonably well.
If you want to run rigid links, you either need to adjust the suspension geometry (parallel links, or triangulated) or you need to remove one of the control arms. When you remove one control arm, only the suspension arms at one side of the axle will dictate the angle of the axle (thus no binding). Meaning the torque/braking loads will be counteracted by only one side of the suspension. The side with both links. How well this could function depends on a lot of factors and probably wouldn't work well in an otherwise OEM set up. Triangulated is widely regarded as the best solution (deletes the track bar) but is also the hardest to pull off in an otherwise OEM set up. Aftermarket may offer solutions to run longer and more parallel arms (requires different frame/axle mounts) as a good solution and alternative to triangulating links or deleting a link.
Disclaimer that this is not professional advice, just take this as my observations. Much of this can be found and explained in engineering vehicle dynamics books.
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dealer says? 
There is a shitton of stuff that goes into a proper link setup. I was dabbling into figuring it all out when I was going to long arm my TJ. Just when you think you got it figured out something else rears its ugly head that you need to work around. It gives me a headache just trying to figure it all out. 
3 links, 4 links, anti squat, anti dive, roll center, instant center, etc.
