Front diff service caused a hum in 4 hi

Howdy Taco world. I recently filled up my 2015 diff and transfer case with amsoil

$30 75w-90 Synthetic Gear Lube - 1 Quart (Pack of 2)

75w-90. The rear definitely feels good and I noticed .5 mpg difference. Drive very smooth in overdrive. My only concern after this is when I tested the truck in 4 hi everything seems fine but when I hit about 50mph in 4hi i get this humm/vibration that didn't exist before I messed with it. It only does it under throttle, goes away when I lift off the throttle. You guys have any insight for this? Should I just drain and refill it again? Is it likely the 75w90 causing this instead of the 75w85 from Toyota? No telling. This truck has 100k miles. I think the previous owner rarely used the 4x4.

One weird thing I experienced when refilling is it is specd to take about 1.6 qts. Well when refilling it seemed to take more than that until it finally leaked out.

I have read about the needle bearing as well. Not sure if that's my case or not. My truck was originally lifted with a strut spacer in the front and a 1 in block in the rear. I removed all that and now have the back stock and ome 883 springs. Approximately 1.25-1.5in lift now. I'm not sure if altering the truck has changed things as well. Lots of possibilities here.

 

I've only ever used 75w90 on my customers Toyotas, its fine, you are likely over thinking it (like your 0.5 mpg gain)

Why are you using 4hi for 50mph in texas? Poor weather?

Needle bearing is usually moderate speed 30mph in 2WD only. 4WD makes needle bearing symptoms go away.

I'd start by greasing all your fittings and a good visual inspection of the under carriage.

 

Well to be fair I have a consistent commute. Always get 16.5 mpg no matter what. Got 17mpg after changing the fluid. Anyways that doesn't matter.


First off the speed limit is 70mph on most of these highways and side roads are 45-55mph. It's been raining and been really cold no freezing though. I didn't think it mattered how fast I went in 4hi. I tested it when I first got it and went well past 50mph just to run it in 4hi to keep it functioning properly, there was never any noise no matter how fast I went. To my understanding it's the trucks that never use the 4wd system that have issues. So it's not the needle bearing because it only makes this noise in 4hi. I'm mostly using it just to prime it and see if everything is good. I will be in Colorado for all of feburary and just want to be sure the truck is good. It would suck if the 4wd broke when there.

What is there to grease? What can I visually inspect?

 

Grease the U-Joints. There should be 5.
Visually inspect all moving parts.
Inspect the CV Boots, bushings and any electrical, fuel, or brake lines.
The owners manual has a whole list of things you can inspect.

As for the hum. You could possibly pull the fill plug and see if it slightly low.
Some people top off the fluid after a short trip or 2. Some just call it good on the initial refill.

 

Thanks guys. Going to get under it tomorrow.