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Ok, here goes. 79 F250 4x4, 4spd, new driveshafts. Driving yesterday, got on the highway up to 50mph, started vibrating like the whole truck was gonna explode. It's not coming from the front end though as no tire shake was evident. At first, if I pushed in the clutch, it would go away. Soon, that made no difference. I got irritated and ran it up to 75mph hoping that something would just break (immaturity rears it's ugly head). Nothing broke. Pulled over, revved engine. It's smooth.
My thoughts would be...Previous owner rebuilt trans. Maybe he mucked up the front bearing? Pinion bearing? Rear hub bearing? Ring and pinion?
I just did brakes and installed new drums. It has dual tires on rear.
Originally posted by 460f250 My first thought was u-joints, but you probably put in new when you did the drive shaft. Is the new shaft balanced?
Just a thought.
Both shafts balanced. It didn't vibrate when I test drove truck or brought it home. I did the brakes. It sat all winter and I took it out yesterday.
when I got my u-jounts done on my old truck the person decided to put my driveshaft in a vice. I drove it and at higher freeway speeds it would vibrate. It came to be that when it was put in the vice it got bent just a little bit and that was all it took.
May be a long shot but.....I had a similar problem and it finally (after lots of trial and error) turned out to be the ignition module. The truck would be misfiring so bad I thought it would blow, and then for no reason it would be smooth as silk again. After changing many components, and finally the module, it hasn't missed a beat since. If you're sure it's mechanical and not anything else then so be it, but if you're not sure try a spare module.
One of the bolts holding that new driveshaft could be coming loose. Ford replaced the U-joints in my van once and forgot to tighten the carrier bearing bolts.
The ignition module can be tested for free at most parts stores if you don't have a spare.
I located the problem, I believe. U-bolt on front of rear driveshaft was loose. Well that little blast down the highway surely did it in so new U-bolts and a new U-joint to be on the safe side.