And it's official: Bad u-joint. Forward joint wasn't terrible, but I changed it anyway. Aft joint, however was another story. A definite 'flat spot' if you will, in the joint. When that flat spot played, it caused a vibration that resonated throughout the drivetrain. It was very obvious when I flexed the joint manually. Change the joint, test drive and the proof was there. No more wiggle.

 

NewOldTruck,

You still might want to check out that drive shaft. An out of balance drive shaft, though sometimes not noticable, can result in premature failure of the U-Joints. I'm getting the shakes too at 50+ mph, but all my joints seem to be good. So I'm going to take the shaft in next week to get it balanced. I was qouted a price of $35. Not too bad considering.

 

i found my problem was rear shaft u joints as well. got a replacement set and went under the truuck to start and tried a 1/2 " socket and the the pinion moves for a while before the shaft starts to move. definite wear in the u joints. it looks pretty bad. but as u know thats not the right size socket and i dont have 12 mm in half inch drive and they are rusted as hell on there so i need to go find a 12 pt 1/2"dr impact socket to break them free. i soaked them in kroil so hopefully i dont break the head off of any of them. does anyone know if they are loc-tited in from the factory? if so i may need to apply some heat to break the bond. so i will let u know how it goes when i get the right tools and time to fix it.

 

yup that did the trick. front one was fine so i saved it in the spare trail parts bin. rear one on the other hand was toast i cant belive it was still on the truck one of the nipples was egg shaped with no needle bearings left in the cup and the cup was egg shaped and missing some metal and there was just rusty powder in the bottom of it. as a helpful note for anyone else doing this task the bolts that held my yoke to the pinion flange were loctited in and a little heat broke the bond and i was able to zip them out. also auto zone and pep boys both tried to sell me the wrong size ujoints. i bought the wrong ones from auto zone last week and then had to go to pep boys in the middle of the job to get the right ones. i took an old one with me and made sure to get the size right. i should have done this job a long time ago. its much quieter and smoother. next project is to try to take the vagueness and mushiness out of the steering. i have already changed all the bushings and springs in the front so i guess i will do rod ends next along with brakes and rotors and bearings

 

Since you've nailed it to a U joint, not for the OP; but others might benefit:

I have had (on some other sort of truck ) similar, very speed sensitive oddness/vibration which was difficult to locate. One time was tie rod ends, did not find for a long, long time, when it finally got very bad, and very scary to drive.

Another time was 4wd front wheel u-joints. Also got very, very bad to drive.

And another was the (more or less tie rod ends) of the steering arm. Lots of slop when finally obvious enough to diagnose.

All started as minor things that happened sometimes at some partcular speed range, often with "speed plus a bump", such as a bridge joint. All slid past mechanics multiple times before getting bad enough that they (or even I) could figure them out.

Some of these were aggraviated or appeased by swapping tires around or rebalancing tires, but I think this was often more "rebalancing the tires putting off finding the slightly bad component a bit longer" than "rebalancing tires actually _fixing_ (or being) the whole problem."

 

FYI: The only time you will ever need a front end alignment is after parts have been replaced.