My 2019 4Runner (113,000 miles) makes a weird humming almost grinding noise when at high speeds in 4wheel high. It is intermittent in terms of it happens every other second, almost rotational at about 50+ mph. It does this sometimes in 2w high at high speeds as well, but more noticeable in 4H.
Only happens when foot is on gas, goes away when I let off gas. Also seems to not be noticeable when pushing gas pedal hard, only when foot barely on gas.
Thinking bad U-Joint, differential, or something else. I put new rear diff fluid in a few months ago. Recently put new T-Case fluid, tranny fluid, and coolant.
It is a very bad idea to be driving at highway speeds while in 4 wheel drive. The system just isn't made for that. I would open you owner's manual and read what it says about the use of 4wheel drive, low and high. IMO, if you are able to go highway speeds anyway, why do you need to use 4wd?
You can definitely go freeway speeds in 4H. Technically you're only limited by redline in 5th gear (not that I'd recommend it). However I agree that if you really NEED 4wd then you shouldn't be driving that fast, but there are situations like if you're going between patches of dry pavement and snow on the highway (which is what I was experiencing), or if the main roads are mostly clear, but the surface streets are packed snow. Just don't make any sharp turns in 4H on dry pavement....
Manual also tells you to drive 10 miles a month in H4. Many people seem to meet that by doing straightaways in the rain.
In what conditions are you driving in 4wd high at high speed?
On the highway in snowy-ish conditions (wet road with slushy slick spots). Was slick in spots on the highway so just left the 4H on the whole trip (about 30 mins).
Keep in mind that four wheel drive does nothing to help you if you can make it up to highway speeds. What helps with slushy slick spots is really tires when you are at highway speed. Four wheel drive can help get you going if it is really nasty, or up a hill. If yours has awd capability, or "full time 4wd" I would use that instead of locking your center diff (terms could be wrong for yours). My GX is full time 4wd and that is great for icy conditions, and if I get in a really nasty spot or hill, that I can't otherwise get up, I lock it into real 4wd.
As @doc mentioned, check your owners manual on the use of 4wd for yours, it would be a shame to damage the diffs or transfer case on a nice truck like that.
My 4WD is part time in the sense it has to be manually changed from 2H to 4H (it also has 4LO, which I would never use on highways! off-roading only.) 4H can definitely be used at highway speeds, just should not be used very often unless slip is expected (which is what I was experiencing in 2H so switched to 4H).
It doesn't make the noise when just coasting, which is what bad wheel bearings would do. Only makes the noise when my foot is on the gas. Most noticeable when foot is barely on the gas. Goes away the second I release the gas.