Strange shaking at highway speeds and lopey idle
 

While idling, most of the time my Blazer will randomly act like its about to die, but then smoothes out again. It'll do this 3 or 4 times and then run sorta smooth for a while. Replaced fuel filter and pump, air filter, oil, spark plugs and cap+rotor. Only thing I haven't changed yet is the plug wires. I also took it on the highway for about 12 miles (my first real drive with it) and I noticed that it shakes a lot around 40-60 MPH. you can feel it inside the truck and in the steering wheel. I have no idea what would cause both of my issues......