shakes badly at 40 miles per hour, if i speed up or slow down it stops, all the time
front end shakes badly at 40 miles per hour on 1993 Ford F-350
If this isn't a dually have the tires rotated and balanced as a out of balance tire will always come in and out of balance at a certain MPH, Then if it persists you've eliminated that possibility and have the tie rods and suspension checked
check the sway bar busshing or the track bar busshings
try changing your tirod ending i did that in my dually and it drive fine now if you let it go it will destroy your ball joints
check your front shocks, it will cause the same thing.