I have a 1994 2WD 1/2 ton Chevrolet Suburban that is having a strange issue. It is an automatic with overdrive and has about 160,000 miles on it. Going down the highway running about 1700-2000 RPM it has a bad vibration, If you speed up or slow down it goes away. I have replaced the U joints, plugs, wires, cap, rotor, repacked the front bearings, changed the trans filter and flushed the trans with no real signs of more than normal wear, but I am still not having any luck resolving this particular issue. It is very strange that late winter I was pulling an enclosed trailer 6' X 12' and got myself stuck, (vibration was noticeable before this incident) while atempting to get muself out of the spot I had to put it to the floor several times winding it out but not over revving the enging to cause catastrophic problems. After becoming unstuck i had about a 40 minute trip home and the vibration had went away. It didn't take much longer than 35-45 miles to start vibrating again. I am curious if there is anyone else that has experienced the same issue and can help me with this problem... Thank to anyone that can lead me in the right direction with this matter.

 

Long shot but, when was the last time you have your wheels balanced? It might be that. Or even mud (or sticky debris) stuck on your wheels in the inside would cause imbalance, check that out too.

 

When I first bought my Suburban and while I was driving on the fwy, I noticed a vibration. It was a U-joint.