death wooble, tried everything...HELP?

[lofly'a]

i have a 2000 Silverado reg cab v6 with 123K. about a year ago i noticed shaking that got progressively worse as speeds increased. this was/is an intermittent problem that does not always happen. since then i have change all ball joints, inner and outer tie rods, shocks, tires, checked and rechecked balance, & rotated tires. i rarely drive the truck over 50mph and this only occurs over 50. i drove it to the pick and pull 2 days ago and thought i'd lose my teeth on the way there (about 5 miles). on the way back the ride was as smooth as and old Cadillac. By definition from what i understand that death bobble is the uncontrollable rapid back and forth motion of the front wheels at high speeds. This was confirmed by a passenger that was willing to hang out the window to observe this at speeds.