Granted, I've only read about death wobble, but I've read a lot. And talked a lot. That being said, take this with a grain of salt
You guys hit the common issues, but caster is often the root cause. A lifted truck without a cut and turn done often has it's caster out of spec, or just on the edge of spec. Combine that with worn parts, and death wobble ensues. Poor caster is very common on most leaf spring trucks. Especially if the axle has been turned at all for driveline correction.
Also, I know Marco has replaced everything, but look into the bushings on both ends of your springs.
On my buddies 87 4Runner with a SAS, he still had death wobble, even after brand new TRE's and new bushings in the leaf springs. Much of the issue can come from a lack of inner sleeve on the bushings. Some lift kits don't come with an inner steel sleeve in the bushings which is necessary for proper driving and keeping the spring immobile. Without the bushing, you just mash the bushings together, and they get destroyed quickly, and there is no support in the bushing for the spring to ride against.
And, be glad you guys have the 555 stuff. I used to burn IFS TRE's in a few months, and that was on 33's. 35's, and I lost a new one within 3 weeks.
Baby Beast 2- 1999 4Runner SR5
Baby Beast -1987 4Runner SR5-Gone, but not forgotten