The truck is a '79 F250 4X4 extended cab. I have installed a '79 F350 Dana 60 front axle.

Around town and on short drives there is no vibration. When I get out on the interstate after about 40 miles it will start vibrating when I hit any kind of joint or minor bump in the road. I don't think it is the dreaded "Death Wobble", I have had that before and there is no comparison of that and what I am experiencing. This is almost like driving on the rumble strip, only no where as rough. Last time it was happening, it had started to snow so I pulled over and locked the hubs in. The vibration went away and did not return the rest of the trip. I never had the transfer case in 4 wheel drive, just the hubs locked in.

Any idea what to look for? I had myself convinced it was rear axle bearings and replaced them, but I still have the vibration after about 40 miles of driving.

 

Being that the vibration stopped when you locked the hubs tells you where to start looking.

 

Thought I'd post an update with what I found. I have looked on the net for other problems with different vehicles but there is rarely someone who will post the solution to the problem once it is found. Usually there is some poster who will reply with no useful information as I believe there was in this case. Since the vibration stopped when the hubs were locked in, what exactly does that point to? Transfer case? U joints? King pins? All of these things and more could be affected by the added drag locking the hubs in would induce on the drive train.

I ended up pulling the front drive line and drove it for several miles and had the same vibration. That eliminated in my mind anything to do with the transfer case. I then started to pull the front axels to check U joints when I found the front wheel bearings on the passenger side were badly pitted and rough. They were bad enough I should have had the vibration all the time, not only after I had driven several miles. The other side only needed re-packed and put back together. I will never understand why the vibration went away with the hubs locked in, as the wheel is still riding on the same bearing. Long story short, my problem was bad wheel bearings on the front axel. If you have intermittent vibration, one thing to check is bearings.

 

First, thanks for reporting back to confirm.

It's really hard to remotely diagnose someone else's problems based just on their description. Most of us would go by something similar to what we've experienced. Was your "vibration" more of a rumble or a wheel shimmy?

I believe stuart1 pointed out what might have been obvious, that the hubs were where to start looking, which you did.

Just as reference, I once had a wheel bearing that failed, and the symptoms started out as a vague whine that cam and went, then progressed to a rumble that came and went, always at higher speeds. The rumbles finally became continuous enough for me to locate it, but advanced to grinding noises as I was trying to buy a replacement.