So I just got a 2003 Ford F-250 superduty and when it get up to 50 -60 and you hold it there it shakes not like a super violent shake but enough to make you be like this doesn’t seem right any ideas on what it could be there is a transmission line leak but that is being fix thanks for your help

 

There are some things you should check or have checked.
Things like ball joints, tie rod ends, all things that move when you turn the steering wheel.
Then there is wheel balance, possible tire problem wheel bearings excessive ply in steering etc., , all these things need to be checked out and confirmed good and not worn , loose or defective..