Are the adapter plate bolts tight to the frame, and box bolts tight to the adapter?
There are 2 adjustments you can make to the steering box. One is bearing pre-load (which is the 1st one to do), then thebacklash adjustment.
Is the pitman bearing in the box tight, or sloppy?
Pitman arm tight on the shaft?
Tie rods all good?
Drag link could have worn joints too. Mine weren't horrible, but was enough to give me death wobble a few times. Replaced it, and problem went away. also replaced the upper king pin bushings and springs. All tight now.

On the W250, turn the steering shaft by hand, and see if you have any frame flex at the steering box. My 92 has it pretty bad. Didn't see any frame cracks either. Can buy/make a frame brace.
One other thing to check, if you haven't. Grab the steering shaft up under the master cylinder and give it a shake. Bushing in column could be shot as well.
NEW- '82 CREW cab dually. Cummins, NV5600, 205, D60/D80.

Work in progress......
'93 W350, CTD, ext cab, 5spd., HTT modded H1C, 4" exhaust, Kelderman single bag. 537k and counting!
SOLD- '92 W250, CTD, ext cab, 5spd
SOLD- '99 3500, SO CTD, 5spd
Traded off- '97 3500 CTD, 416hp/892tq.
Obama loves America, like OJ loved Nicole...................