First I'm getting a fairly loud clunk from the "rear axle area" but really only during the shift cycle from forward to reverse or vice-versa. During stop and go cycles like from a stoplight I don't get the clunk.

Secondly I start getting a vibration in the 35-45mph range that goes away once I reach the 50mph+ point.

Other than a cursory look underneath taking a shot at something obvious, everything appears normal except for some slight seepage from the oil seal on the differential where the yoke extends out.

If it's the universals would I be able to tell by putting it in neutral and trying to rotate the shaft by hand on these? What else should I be investigating to help narrow down the issue?

 

The leaking oil seal unfortunately indicates that the rear pinion seal and rear pinion bearing are gone. When the bearing gets loose, the leaking seal is the first sign. The rumbling noise or clunk is another indication. If just the seal was bad, you would not have excessive noise. If you put a 24" pipe wrench on the yoke and pull the handle back and forth, the yoke should not move up and down at all. A bad pinion bearing is also very hard on that rear joint because of the excess movement when driving. Larry

 

I looked again at the what I thought was a leaking seal but it seems to not be really leaking though there looks like some slight old oil residue mixed in the coating of road dirt.

I put it in neutral and was able to get a slight bit of movement of the drive shaft with my hands and the yokes at both ends moved with it. Didn't really see any movement or looseness at the u-joints. Also to correct my OP, it only clunks when putting it into forward gear but not reverse.

 

In your first post you said that you had vibration while driving. That could be wheel bearings, u-joints, rear pinion, tire with a tread separation, tire with a flat spot, tire with a lost balancing weight.
Since it is happening while not in 4WD (I am assuming) that eliminates the front axle u-joints and the front driveline and differential. It might be worth your while to pull the rear driveline and feel the u-joints for roughness or strange feel. Jack up the truck front and rear and check the wheels for excessive play or growling while hand turning the wheel. Confirm that both hubs are in the unlocked position. Place the rear up on jack stands, crawl under truck while someone else starts the truck and puts it in gear. Use a stethoscope at the rear bearings, differential, carrier bearing.

For anyone else reading this, jack stands and blocking the front tires both ways are necessary to assure safety. When the rear is jacked up, the parking brake and transmission park function are obviously not functional. Larry

 

The same goes if you drop the driveline.

You can have perfect u-joints and still have vibration from the driveline. The driveline can be out of balance or out of true.

Is your truck lifted?

 

I'm betting that its the U-joint at the rear. I had no clunk until last night but had a vibration from 50-40 while slowing down for a month or so.

It was the rear U-joint at the rear. Changed both of them on the rear shaft and everything is fine now (Thanks again for the help Doug). Can't always rely on the play in the U-joint while in neutral mine didn't have any. But I do know my truck needs new U-joints every two years. Too much power I guess! LOL

 

Correct there's the vibration at stated speed. I have had to change the needle bearings up front twice due to the seals being shot which consequently eliminates 4wd on the fly due to lack of vacume. That was a problem ($$) I had hoped to settle later this year, and now this.

The previous owner had stated when I bought it back in 2004 that he had to change the u-joints once before. He attributed it to the salted winter roads of White Plains NY while using it as his daily commute vehicle. I bought it 4 yrs old with 114k miles and It's only up to 152k today.

I'm hoping it's the simple fix of u-joints and maybe a fresh set of needle bearings upfront to eliminate those two relatively easy & inexpensive fixes.

I've done universals on older cars but never the big trucks. Are there any surprises or tricks to it besides the shaft appearing pretty heavy, to R&R'ing them?

 

Go to auto parts store with tool loaner program and rent ball joint set. 130psi of air on impact gun to the screw drove them right out.

 

Quick follow up and closure on the post. Luckily it turned out to only be the rear universal of the rear driveshaft. Replaced both ends and greased up the splines under the bootie. All above described symptoms went away.

Now my next project will be to replace ball joints, all seals and needle bearings etc....but that will be another post after I do some research on here.

 

Nice job and thanks for posting your results. I had a clunking/clicking from a stop and the u-joints "looked" and felt fine with no slop. Eventually, I removed the driveshaft and the rear u-joint at the axle fell to pieces in my hand.

 

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