I have a 2001 ford ranger 2wd and i am experiencing vibrations at different speeds and times. I just took a 300 mile round trip on the interstate averaging about 70 mph. I knew my alignment was off so i took it in for an alignment half way thru my trip and I was told that before an alignment could be completed that the lower ball joints needed to be replaced, they also stated that everything else was good.. I replace the ball joints the took it to Sears and had 4 new tires installed, balanced and an alignment done. Alignment is good it goes down the road nice and straight. Now as i am driving the ranger will drive nice and smooth for a couple miles then I begin to feel vibrations (but can't seem to pinpoint where it is coming from) then the vibrations go away and it rides nice and smooth for a few miles then the vibrations starts all over again this continued all the way back on my return trip.

Does anyone have any ideas at to what is going on. It's wierd that it comes and goes every few miles.