I agree with Roamer, I've had front end shake such as you describe in many vehicles, in many it was strongest at some speed, but disappeared at other speeds. I have a theory (resonance) as to why this happens but beyond the word in prens, I don't wish to expound just now.
In my cases I found various causes, From simple tire imbalance to bad alignment to bad suspension components.
In any case, do get it fixed, some of those are SERIOUS problems if allowed to continue.
I have also heard of tires that were "out of round" causing such a problem, and I've had other tire problems do it (What happens when you have spent 40 years driving older cars, you get experience with problems)
I would guess some tread problems can cause it too
Suggestion is to follow this path. Step 1, Have the tires checked and balanced by a good truck tire shop
If that fixes it.. Great, if not, have a good front end specialist look at it (note, most good tire shops have a front end specialist because front end problems cause tires to fail faster)