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.