Greetings! I have a 99 F350 PowerStroke with a vibration in the front end that only shows up intermittently. She has 300k miles on her and I am pretty sure the front, rear axles are original and the transfer case. The transmission was rebuilt around 70k miles ago and seems to in good shape.

The problem: I am driving at highway speed 40+ MPH, in 2WD, hubs unlocked ( original hubs replaced with Warn manual a year ago ). At random times I will get a vibration in what feels like the front that I can only describe like when you drift onto the rumble strip on the highway or something, but a little less intense. I can feel it in the floor, steering wheel, etc. It's also audible in the cab.

I have figured out I can make the vibration stop by flipping it into 4WD. As soon as I slip the switch, it instantly goes away. But it will come back again, typically within a few minutes. It's not consistent, but sometimes it seems like the vibration starts when I hit a medium sized bump or pothole.

I also recently found that if I leave the hubs locked, the problem goes away.

Other info — about a year ago 4WD was not working, the drivers side front wheel would not lock/engage. I ended up having Ford replace the torrington bearing, which fixed the problem. Not sure if that is related at all, but I did have the vibration before that.

This truck overall in good shape, not a daily driver. It gets probably 5-7k miles a year.

Appreciate any help / insight you can provide, but I'm leaning toward "it probably needs a new front end" at the moment. :\

Thanks!