Any thoughts on the source of this noise?

https://youtu.be/IRO15ghkP5A

98 SR5 4x4 Manual with 208k miles

I purchased this 4Runner about a two months ago and thought the humming was caused by the nearly bald tires. I replaced them and turns out the road noise was just masking how loud it was. In the video, you can hear the noise at about 0:40 and on. It is noticeable after about 40mph. My thought is either wheel bearings or the diff/transfer case is low on fluid. It is definitely coming from the front of the car. Both are time consuming jobs and it's cold out so I'd like to only have to do one them.

A little bit about the car: I have full maintenance records dating back to the purchase of the car. The last time I saw a diff/transfer case/manual transmission change was at 99k miles in 2007. I don't know exactly what was changed; the maintenance record just says "gear oil". It's definitely on my list to change it but I was hoping to wait until spring. I plan to take one or two ~4hour road trips in it soon and would like to know if this needs to be addressed now or can wait.

 

I had a very loud humming coming from my 2005. It was also around 40 mph and higher. Changed out both front wheel bearing. Worked like a charm. Obviously can't really tell if that's what yours is but if it's humming my guess would be wheel bearings. I would suggest changing them both if you do change them.

 

does the hum go away when you make turns?

 

No. But given that the hum is only present above 40mph, they aren't going to be very sharp turns. I was driving in the mountains yesterday and the noise does not change when turning.

I've posted to some other places about this issue and so far we have:
- Pinion bearing
- Wheel bearing
- Transmission problem

I think I'm going to tackle the pinion bearing first; I can at least remove the diff myself and then take it to a stop to press out the bearings. I think if it was a wheel bearing, the noise should change when turning.

 

There's another thread with similar noise issues. I'd check the U-joints in your driveshaft. Mine went bad last summer and made a similar noise.