Front drivetrain vibration

[kobresia]

It seems like it can jinx things to think about how reliable a vehicle is, which I was doing recently for my 2004-- it's had so few issues and even the original battery is still going strong. I even started replacing some parts for cosmetic purposes.

Well, it's developed an obnoxious, relatively low-frequency vibration/shudder, but only when the front axle is engaged (it's a V6). I thought it might just be the u-joints, it had been a while since I'd greased them and they did not accept grease very well, so I figured it was time & they were likely the cause. After replacing them, it's persisting. I'm starting to suspect it may be a differential problem, but I'd hate to start replacing parts only to find it's the transfer case or something.

The vibration starts at 30 MPH in 4-Hi. That's about the limit in 4-Low, and the only vibration I noticed in Low range was from the high RPM of the engine.

There were a couple of incidents a couple of months ago where the 4wd made an awful screeching/grinding noise while engaging on the fly when I didn't take the load off the drivetrain quickly enough for it to engage within a few seconds, is it possible (or likely) that the actuator jammed the gears and broke something? What should I focus on next?

 

[kobresia]

Small update: I have noticed that when the front axle is engaged, there's a squeak all the time when the vibration isn't noticeable. Not grinding, just a squeak that sounds like a dry u-joint or something, but now that the u-joints are new & greased, that probably isn't the issue.

Could anyone offer insight into how the actuator works with the front drivetrain? How likely is it that one of its components got bent somehow and it's interfering with one of the shafts or gears?

Would I be correct in guessing that the main ring & pinion and planetary gears in the front diff are turning whenever the vehicle is moving? I'm thinking the next step in problem isolation is to know which components are engaged all the time vs. only when actuated.