High Speed Vibration

Ok going to explain this the best I can.

My '94 Mighty Max has been shaking at high speeds. It drives great around town
but it seems whenever it is under load (on an incline, or occasionally straightaways)
at a speed greater than 65 mph the whole truck shakes. Taking my foot off the gas
makes the shaking less intense, and pressing my foot down on the pedal makes the
truck shake worse (there is also a low chugging/rumbling sound). When I release the
gas and drift down to a slower speed (55mph) the shaking disappears completely.
This makes me think it is either drive shaft or transmission related - and not tires. In
other words, the gas pedal is directly related to the vibration.

Things I've replaced: Ball joints, tie rods, sway bar and strut rod bushings, fuel filter,
distributor cap, plugs, air filter, brake pads and rotors.

Front suspension is tight. Still shaking.

Now how do I test for play in the drive shaft? I see no leakage and I can't turn it or
detect any movement in the u-joints or yokes with the truck off. Is there a more precise
test I can do?

Jack it up. Look at the center bearing. Make sure it's not collapsed into the rubber. Then grab and pull the U joints. look for any movement. Twist the drive line just slightly back and forth. Look for and movement that is not conected. One part moves before the other.

Quote:

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When I release the
gas and drift down to a slower speed (55mph) the shaking disappears completely.

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This is almost definitely a u-joint issue. Camoit gets you started on the inspection. I would suggest dropping the drive shaft for inspecting...get some PB Blaster started soaking on the differential bolts that hold the shaft in place before you try to loosen them.

Good idea to check the transmission mount. The rubber gets old, and is under a lot of stress. (For that matter, check the motor mounts, too).

Replacing the u-joints is not a difficult or expensive task, but it does require care, so as not to damage the drive shaft's balance. If you replace the u-joints, it's a good idea to replace the transmission mount...and visa versa: if the mount was allowing the shaft to flop around, then it caused some wear.

I had my u-joints replaced and the shaft balance tested at a shop. I got a new mount, and the absence of vibration was wonderful. DIY= about $50 for u-joints and mount. Shop charged me $125 for replacement and spin'test. Mount was about $12 through RockAuto.

Check tires for broken belts or lost wheel weights, went through the mountains in AZ years back and broke a few belts on my tires and it was a shake-fest until I got them changed out.

I had the same issue with some worn tires. Got new ones and had truck aligned, been fine ever since. I got the wobbles when it was above 65mph