I had the same thing on our Orange Zuk. Death wobble at a certain speed range.
-I had the tires rebalanced at 3 different shops
-Rotated the all the tires around
-Changed the kingpins and bearings
-Changed out the wheels bearings
-Changed out the hubs to brand new factory ones (changed axles at the same time too for the heck of it)
-I actually changed out the entire axle housing and put all new components on it
The only constant was the tires and wheels, though I'd exhausted efforts there (Centerline HTII's with TSL SX Swampers). I finally decided it was nothing mechanical and could not be fixed. It was just an evil demon that had decided to posess my vehicle. Not being a religious person, I felt spending money on an excorcism might be a waste.
I finally cried 'Uncle' and threw a panhard on. It was difficult too I might add because of the VW TD and the shape of its oil pan. I made it work though, and the vehicle has driven like a dream ever since.
The demon probably still lives in the vehicle, but he's also probably really pissed off that the panhard bar locks out his little fun game of Death Wobble.