Vibrations at 40+ mph

[drase]

I have a 2wd, 6cyl, auto, 87 MJ. I have vibration above 40+ mph only why I'm accelerating. If I let off the gas vibration stops. My mechanic checked the U joint and said it was fine. He said the timing is off and could not take the distributor cap off as its seized to the engine. He advised for me to put premium gas in it to see if the vibration stopped. I did this, and vibrations did not stop. We put new plugs in, and checked all electrical connections.

 

Does anyone have any idea what it could be? I'm thinking about taking it to another mechanic for a 2nd opinion.

[Eagle]

You definitely need a second opinion. The timing is not adjustable, and there's nothing inside the distributor that would affect it anyway. And since the distributor cap does not attach to the engine, I can't understand how the cap could be "seized" to the engine.

 

The vibration you describe sounds like a u-joint vibe. It might not be a bad u-joint. Is the axle original? If an XJ axle was swapped in, the u-joint angle could be wrong and that could create a vibration.

[drase]

Yes, the axle is original. Everything on the truck is original. I replaced the hoses, they were rotten on the inside.