Hey guys, I've been trying to diagnose this issue for a bit and have a few idea's but wanted some more input before I start wrenching and buying parts. I have an 02 qx4 that has a pretty nasty front end vibration above 55-60mph but ONLY when in the auto mode and the front wheels are engaged I have a slight vibration from 50-55mph in 2wd mode, which might also be related since I know the front hubs on these trucks are always engaged. Symptoms are a high frequency (feels faster than wheel speed) vibration under acceleration, no vibration on deaccel. Some reading points to things like the front U joints in the driveshaft being bad, though I wasn't able to feel any slop in the shaft when I crawled under my truck. Wanted some opinions on what else to look for. Plan to change the Tcase fluid soon since that'll be the simplest thing to do and I'm sure it hasn't been done in a long time.

You might want to get a sirometer, measure the frequency and see what it corresponds to at your ratios in top gear. That can eliminate a lot of guesswork about where come-and-go vibrations are originating.
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That's a great idea!

I have diagnosed mine toward either a bad transfer case output bearing, differential, or front driveshaft. When I remove the front driveshaft the vibration goes away. I am leaning toward it being the front output bearing of the transfer case, because I can feel some slop on that flange, whereas the diff flange is solid.

My front control arms and ball joints were shot. I replaced them and most of the vibration has gone away. I suspect I could use new front diff bearings, front wheel bearings, and new struts as well.

I was wrong, the vibration was still there. Mechanic found a loose wheel bearing and tightened it up. We'll see how it goes.