F 250 knocks loudly and vibrates on high speed turns

Have new tires, had them rebalanced, new shocks, alignment. Drove 225 miles from Dallas and had severe vibrations on very long high speed turns on turnpike and interstate highways. Checked into Ford shop, they added a steering stabilizer (?) to it....no good. They placed it on rack, found rear shocks were blown out ( no towing, rough roads, etc). Replaced shocks..same stuff
test drove it with mechanic in the bed, he now says the noise is coming from my tool box...losing patience with their mechanic.
The vibration starts at high speed, 55+, is worse when turning to right. It occurs on long curves and gets more severe until it feels like someone is knocking under my seat to get in.
I can understand a noise coming from my toolbox, due to the vibration, but to say that is the cause?? have lost confidence in Dealership. need help, feel unsafe now on road. They say all front end components and everything else underneath looks great. have 86K miles, 4x4 extended cab. nearly all highway and city miles.

are you pushing the break when it shakes or is it all gas and no break.
If it is all gas and no brake then it could be power steering

all gas, on cruise.
only does it on wide long turns. very scary.
no ideas?
dealer is stuck.
heard from one other guy...has same prob...no solution.

have you check to see if the transmission might be loose or any other part. My 81 f-250 use to do that for awhile then i found out that my transmission was lose and that fixed.

We were having a similar problem, the rear stabalizers wseere loose by half a truns on the lowere bolts ans caused an awful knocking. Knock them with a rubber mallet, if they rattle at all, theres your problem.