Truck is a 2WD, 79 F-250, 351M, 4 speed manual, Dana 60 differential stock as near as I can tell. I've got driveline vibes that are strongest @ 50 mph. I've replaced all three U-joints and the carrier bearing feels like new. If I have the rear shaft out and run it up to 50mph there are no vibes.

Any guesses?!

 

have the driveline checked for balance

 

Palm,
I've got a dumb question.
How do you get up to 50mph when the drive shaft is out?
Can you describe the vibe more clearly?
Does the vibe cycle? Is the frequency high or low?
Does it shake the steering wheel at all?
Does the vibe come and go?
Does the shifter move around when you go over a speed bump or curb?
Have you worked on the rear end at all? Is it a differential, limited slip, or posi?
Does the brake pedal pulse at all during use?
Have you replaced the axle bearings recently?
The generic question, "Why is my truck shaking?" Could be caused by about a dozen different things.
Throw me a bone over here!
KingFisher

 

Answers to dfisher1's questions:
I ran the speedo up to 50 mph with only the front driveshaft connected. (front shaft spinning and the diff disconnected)
The vibe is steady and @ a frequency about equal to the revs of the driveshaft. (~ 4 times per wheel revolution)
No shakes @ the steering wheel.
Vibe is always there with 50mph being the peak. Vibe is not related to engine rpm.
Shifter doesn't move around and motor mounts are solid.
I haven't worked on the rear end at all other than to check lube level which is good.
No pulse @ the brakes and upon inspection are almost new.

Other observations: Yokes @ the trans and diff are tight although there is some side to side play at the trans output. I haven't checked drive shaft angles but assume they are good considering this is a STOCK truck.

 

Does the vibe change if 'n you give it more throttle, of back outta the throttle at 50mph?

Is the pinion nut really tight?

 

Just a thought but you could check for excessive play in your slip yoke. I heard of a trick to temporarily balance a driveshaft never tried it though but it cant hurt. put a hose clamp around the shaft just tight enough so it will stay where you put it but loose enough to turn easily by hand. Then run the truck up to speed and the clamp will balance itself. then all you do is tighten the clamp so it dosent under hard acceleration. If it works then you know its the balance and you didn't remove the shaft for nothing. if it doesn't you only lose about five minutes.

 

That sounds like a cool idea. I'll have to remember that one.

 

Hey, I have a vibe question. Mine comes and goes at steady speed. It starts, gets a little worse, gets a little better, goes away, and then starts over. I figure it is a harmonic vibration between the tires, but what do you guys think?

 

Check to see if it happens when your clutch fan engages. I had a REALLY bad vibration once when decelerating, it turned out to be a bad water pump bearing. So it may be your fan is unballenced, when it engages it vibrates and when it disengages it goes away.

 

It is a direct bolted fan. It is always engaged. It never happened until I put swampers on and shortened my driveshaft(myself). I am not really that concerned about it, just curious. Thanks, Ted

 

Have you guys checked your driveshafts to see if the weights are still on them? If you shortened your drive shaft then it needs to be balanced. I found a shop that made a rear drive shaft for my truck that has no weights. The shop put it on the balancer when welding it, doing this there is no chance of throwing weights. Just an idea if you did throw a weight and need a drive shaft.