Diagnosing a vibration

[got2haul]

1980 short box stepside.  2-piece driveshaft.

When I bought the truck, it had a vibration that began at about 42 mph and continued until about 53 mph (at least those are the speeds at which I noticed it).  It wasn't a violent vibration, just annoying.  It occurred at these speeds with the tires and wheels that came on the truck when I bought it, and at the same speed with a brand new set of freshly balanced tires and wheels.

I had the center bearing replaced recently since it was very worn and I suspected it might the source of the vibration.  The result was that the vibration got worse and begin 5 mpg earlier at 35 mph.  I crawled under the truck and could see that the shop had not installed the rear of the driveshaft in its original phase (I know because I marked the driveshaft and the rear axle flange before taking it to them.)  I unbolted it and bolted it back up with the marks aligned.  The speed range of the vibration moved to begin at 42, just like before, so the center bearing wasn't the culprit.

While troubleshooting, I wanted to eliminate the tires and wheels as the source, so I set the rear axle up on jackstands and blocked the front wheels, and made sure I had a clear road ahead, just in case.  I fired it up and took the speedo up to the speeds I mentioned, and the vibration was still there, at the same intensity.  I realize it could still be the rear tires since I didn't remove them before the test, but I don't really think so, since it occurs with both sets of tires.

Even though I can't determine by sight if either the front or rear section of the shaft has thrown a weight at some time, I suspect either one or both sections as the source of the vibration and I want to unbolt it and have the balance checked locally.

One question I have is: do I need to take the yokes with it to do this?  If so, how do I keep transmission fluid from leaking out the tail of the the transmission while the yoke is not in place?  (WILL it even leak?  I have never removed a transmission before, so I don't know how this works.)

Does anyone have any other suspicions of something else might be the source of the vibration that they can recommend I check based on my description?  Thanks for any help!

[zieg85]

Try taking the tires/rims off and doing the same speed.  Make sure you put the lugs on to hold the drums in place.  Do you still have the oem fan clutch.  I have seen them bad and causing the same symptoms, My $.02

[ccz145a]

Your right, you have eliminated the tires as the source. Furthermore, when you messed with the driveshaft, you affected the vibration. I would focus there.

Change your u-joints, then check.
If it still vibrates, then take somewhere to check balence.

You may lose a little trans fluid when you take the yoke out depending on the angle your truck is sitting, but it won't be much. Just replace it when you put it back together.

You've got $0.04 now.

[VileZambonie]

Check your U-joints first and check your pinion bearing. Pop the meatball cover off too and check for movement in the carrier bearings also. one trick you can try if you determine it's a driveshaft balance problem is try placing a few hose clamps in a few different locations on the driveshaft and it may cancel the vibration.

[got2haul]

I pulled the driveshaft last week and had it rebalanced.  Picked it up Friday afternoon.  Had the u-joint at the transmission-end replaced, as well (the other two had already been replaced recently, when I had a new clutch installed.)  I installed the driveshaft this morning and took it for a drive.  Much, much better, although the vibration is not completely gone.  I was checking angles after I installed it and before I took it for a test drive -- I think the rear pinion angle is low by about 2 degrees.  When I lowered the rear, the pinion aimed way up, so I put in a 6 degree shim to bring its nose back down.  It looked like the right angle at the time but now I'm not so sure.  I'll try a 4 degree shim next week.  I think that will probably take care of it.