Vibration on 79 F150 pickup

I have a 1979 F150 4WD pickup. It has a bad vibration at around 20 to 25 mph. The whole truck vibrates and rattles. The vibration stops if I shift into neutral, so that rules out the tires. I looked at the
universal joints and they are fine. I finally took the truck to a mechanic, and he checked the same things I did (tires and univ. joints). He said he can not isolate the problem either. His only suggestion is a bent driveshaft. He said the only way to be sure is to take the driveshaft to a special shop that works on them, and have them test it. But, he said that there is no such place closer than 100 miles away, and the cost to travel there, plus the testing would probably cost more than getting another driveshaft from a junk yard. But he said it's a gamble if another driveshaft will solve the problem.
Does anyone have any ideas? I am at a real loss on this one, and the mechanic said he is not sure too.....
Is there any way to check the driveshaft myself, if I remove it?
What else could cause this problem?
Like I said, it only vibrates badly at the 20 to 25 mph speed, and of course that is the speed limit in the local rural towns.
All advice appreciated !
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snipped-for-privacy@NONE.com wrote:

if you had a shop manual for your truck( i had the 83 ford, passenger car books(about 5 or 6 its been a long time ago.. they had every type of measurement you could imgine for your vehicle and how to test for it.. in your case you would need the truck series manual... if i remember correctly you could have the driveshaft on the wrong way ( when you remove it you have to mark one side with a chisel to put a dent in the shaft so you will know which side goes up or down and mark the U joint are where it is attached to the rear end yoke..... if its wrong you get a lot of vibration.... also remember something about putting a large hose clamp onto the driveshaft if it vibrates,, cant remember how to determine where to put the srew type part of the clamp(but this is the weight that balances the driveshaft if needed).... i think it was somthing like this to measure if your driveshaft is true or not:: jacke up both back wheels and put car in gear with motor running and in gear and let the rear wheels move( i would not want to get under a car like this, so maybe its not correct or not??? just trying to figure it out from memory????,,, use a piece of chalk on a stick(dowel), probably taped ot it and lightly touch the driveshaft in different places along its length.....then the high spots on the driveshaft will be marked with chalk......dont have nay ohter info, just trying to remember and guessing the rest??? never ever had a problem with this on a driveshaft, but did have problems with driveline vibrations that threw out some 3 speed trans. on a 66 mustang..... years ago.. found out it was the pilot bearing in the crankshaft that was letting the front shaft of the transmission vibrate and this would blow the transmissions.. found out about 4 yrs. later when pulling the engine to have it rebuilt........
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Does it seem like a drivetrain vibtation? or just a general vibration that you could probably get from a rock concert with your truck parked in front of the speakers?
At my house there's a 79 F-100 (really that part is irrelevent)... it had some stock mufflers put on in place of some performance ones, because arround 40-45Mph or so, in 3rd gear, the exaust vibration was at just the right frequency I guess that you couldn't hear anything, the whole cab vibrated.
And you said it didn't happen neutral, correct? I would think this would rule out the drive shaft possibility, because if you're going 20 mph , and then shift to neutral, the driveshaft is still turning.
I guess what I am suggesting here is it might be an exaust related vibration, but I really can't tell since I can't hear it...that would probably be very obvious.
If you drive to the proper speed to make the vibration occur make note of the RPM (if you have a tac of course), shift to neutral, and then bring the engine to the same RPM that it was just before shifting to neutral, does the vibration come back? Maybe it's something that happens from the engine and before the transmission, that vibrates at a certain RPM or something.
Good luck.
Nic

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it is not the driveshaft. it is spinning the same speed wether the trans is in gear or not. sounds more like a trans berring problem, or could be a motor problem.
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