I just saw this thread......
With the adding a lift, suspension or body there is a lot in the steering geometry that changes. However the most signifigant change is the 39.5 super swampers. When we started to go from 36's, 37's to 39.5's and 41's, 42's on the Dana 60's we got the death wobble bad even though we replaced everything with the kingpins.
My friend Jimmy was going to scrap his Jeep he got so frustrated with death wobble. We played with everything. Alignment, steering geometry, dampers etc. Nothing worked. He was on a brand new set of 41 inch Super Swamper IROK radials from 36 inch bias IROK's. I started playing around with it and stacked some big flat washers on top of the kingpin springs. Basically I preloaded the springs for more tension and less spring travel. It worked. We totally took the death wobble out and it was litteraly a 100 MPH Jeep on 41's.
He drove it that way for 3 years and never had a problem after that.
What I figured was the bigger tires put more loading on the kingpins because of more leverage and a heavier wheel and tire package. So the spring needs to be stronger accordingly.
And we did not have adverse wear or any broken plastic cones. And we spent hundereds of dollars to not fix it and $5 in flat washers to fix it.
We have repeated this process on several other Dana 60's in my 4x4 club and it has worked every time.
You may have other issues other than kingpin but if everything else is good then try staking a few big flat washers on top of the springs.