I have a 1999 Toyota 4Runner 4WD Limited with 3.4 L V-6. I had Transmission, Rear Differential, Front Differential, and Transfer case fluids changed out about the middle of last year. Not too much after all that was replaced it seems that there is a sort of humming that I have heard while driving on the highway from speeds of 70-75 mph. Would you possibly have any idea as to what it may be?

 

This only started immediately after changing the fluids, or just at some point after changing them?

The first thing to check is always tire wear. Wheel balance. Driveshaft.

 

The mechanic may not have added enough fluid in one of the gear boxes. You could check the fluid level yourself.
On the rear diff, remove the fill plug and check it. It should be full right to the bottom of the fill hole.
I believe the other boxes can be checked the same way.

 

Yup. I don't know anything about the automatic transmission, but...on all the gearboxes, take out the fill plug and either gear oil will start coming out right away, or it should be to where you can find the level by sticking your finger in there. Just at the level of the bottom of the fill plug, without overflowing, is the correct height. Can always bring it in to the person who did the work, too.

 

The humming is most likely the rear U-joint, grease won't help it now, but there are 5 grease locations on the rear driveshaft, and 3 on the front, 2 of which use a small amount of a different type of grease, could be a wheel bearing, but usually the rear most u-joint, the fluid change is most likely coincidence, but it wouldn't hurt to check, you might want to give the rear diff breather a twist to insure it isn't plugged every oil change as well to avoid future problems. If it is the u-joint or wheel bearing, I wouldn't bother with anything else but OEM parts.