Front end shake

I've got a '99 Trooper in which the front end shakes when you get the vehicle up between 50-60MPH. The steering wheel shake becomes more predominant when turning to the right. When I bought this vehicle last year I had a similar problem that I took care of by replacing the 2 front tires and having an alignment done (drove perfect then) and having all the tires ballanced ( I left the rear tires because there was plenty tread left). About 6 months later it started shaking again so I took it to the tire store and had them reballance, rotate and check alignment. They said the alignment was great and that all suspention components were tight. (With the older rear tires now in the front, it shakes even worse). The guy at the tire store tried to sell me new tires because he said the reason it would shake was because the tires are now not prefectly round... Do I need all 4 tires even thought my tires I now have in the rear have less than 10k miles on them? Is there something else that could cause this shake to happen? The tires I have in the front (the older ones with good tread left) have had 3 nail plugs on 1 and 1 on the other- could be possible that those are shot now, but they look round to me.