Vibration when driving straight

I've got a 94 Trans Sport that I replaced the lower ball joints on in late
November. Tight after doing the job I took a 350 trip on highways and noticed the van shook in the front end when driving in a straight line. A very slight turn to the left or right would make it go away. When I say turn, I mean so slight that the vehicle would bearly change lanes on a highway doing 75. I found during the trip that if I stayed in the passing lane on a 2 lane interstate the vibration went away because the slope of the road forced me to steer slightly right. Since tyhen it has gotten ridiculously worse and shakes and vibrates all the time. It does lessen when in a turn. In a straight line its like going over train tracks.
Any ides?

I assume you have ruled out unbalanced tires and alignment.
We are talking solid low frequency vibration here, in fact, at same frequency as wheel speed. (not humming as caused by bearing).
I lived through that before. Exactly that, every bit of it. CHeck outer tie rod ends first and hope they are the culprit (one is enough)....otherwise the play could very probably be caused by inner tie rod ends, which is a documented issue in several threads here....including one of mine. Come back with results or your investigation for more info if..when needed. replacing inners is somewhat annoying....

Going to check it out in about 30 minutes. I did lift up drivers side wheel, assuming problem is there since I felt it throught through the floor. I grabbed tire and shook it. I didn't notice any play that would warrent this type of shake but I'll get a closer look soon. Its finally sunny and 40 degrees today!

Ater lifting the entire front end and checking play at each wheel it seems that the passenger side outer tie rod was bad. Drivers side feels alright. After replacing tie rod it seems to be better. I'll know later when I take it on the highway at speed. I put a hurting on both front tires from waiting too long. I'll post tomorrow morning.