91' 350 tpi auto trans - I have an intermittent vibration under normal driving conditions. Sometimes it'll be nice and smooth and other times it shakes like mad. No knocking or anything, just shakes the heck out of the car. Just had new tires mounted/balanced/aligned so I know it's not that. Just did big tune up on it, timing, rotor/cap, plugs, wires, oil, etc...

If the balancer is going bad, how do I troubleshoot it? I've checked the U-Joints and they look fine. Pretty sure the vibration is coming from the engine. Help!!

 

universal joint is the first thing i'd check, again.....i had a pickup that did that for a while and I checked everything but that......then the driveshaft fell out...