When I am in Full Time 4x4, I notice that the front diff makes noise when get up to speed and gets a little louder the faster I go.
Bad diff, need some oil or is that noise normal?

It only makes that noise when in 4x4. In RWD it doesn't make a peep.

 

Sounds like you need to check your diff fluid level. If the fluid is too low your gears start rubbing because they're not getting enough lubrication and that results in the noise. It's not going to do it when you're in 2wd because the front diff isn't engaged.

 

I will see about getting the front, rear and transfer case and changed out tomorrow.
Go through the haynes and see how much fluid I need to get a price. I have to change the Buicks oil tomorrow, so I might as well change that out in the Jeep.
Im sure it also has never been done either.

 

Now wait a minute it should be in your tcase cause you have auto hubs which makes everything turn all the time

 

Not if his fluid is low. There's no pressure on the gears when it's in 2wd, hence they don't engage each other. The front tires just spin freely. But in 4wd, the gears are engaged and low fluid won't lube them enough as they come in contact with each other, resulting in a low hum, or even a grinding noise.

 

Gear oil does not make the gears engage

If he does not have locking hubs, the front differential is turning regardless whether the transfer case is in 4wd or 2 wd

The noise you would hear from lack of oil is the pinion and carrier bearings they need oil

 

My front diff fluid has been low for a long time. Temp axle just never came back out. So in 4-wheel noise 2-wheel no noise. Noise is deffinatally comming from axle not t-case. Going through this right now so sorry to prove some wrong but "no rdplx" is right. The t-case was gone through by one of the most knowledgeable Jeep people I know and the fluid is at level. so check your front diff for fluid. Mine is a quarter inch low.

 

I did'nt understand a sentence you typed

 

Dont mean its the exact problem you have ya check the fluid but also be wary bout the tcase i guess i see how torque could make a noise but i still dony think its it

 

Can you describe the noise at all? Maybe its a bad ujoint in ur front DS

 

It is like a hum/whine kind of noise. It isn't noticeable until 35 or 40 mph, and in the 55mph to 60mph you can notice it. It isn't like super duper loud, but very noticeable.
I even shifted to 2WD on the fly and the noise stopped.

 

I never said that gear oil makes the gears engage. I simply said it lubes them. Make sure you read something correctly before you try to sound smart. And yes, the gears are turning in the front diff regardless of being in 2wd or 4wd, but The ring and pinion are NOT in conjunction (touching) when you're in 2wd, thus no noise. They only turn in conjunction when 4wd in engaged. If the front diff fluid is low, they won't receive proper lubrication (2wd or 4wd), and when engaged, a low "humming" noise is created from the gears rubbing on each other, just as the OP has described.
Bottom Line: Drain the font diff, check the diff housing and ring and pinion gears for any scars, chips, or noticeable wear. If all is good, Put the cover back on with some gasket maker and put the proper amount of gear oil in (if you stick your finger straight in the fill hole, the oil should be just touching the bottom of your finger).

 

Why would the ring and pinion move there both stationary

 

^x2 Ring and Pinion touch all the time or they are broken.

 

Sorry, let me clarify. They rotate constantly, but not under pressure (forgot to add that part in). When you put your XJ into 4wd, the front drive shaft is engaged, and it sends pressure on the gears, namely the ring and pinion gears, making them move, or rotate, together under pressure. That pressure is what causes them to rub against each other more harshly. They do constantly touch, just not under pressure, as I said I forgot to type in the other post. I got ahead of my brain while typing, so I apologize. Hope this clears it up for you.

Either way, the OP needs to check his diff fluid level. It's most likely low. It'd be a good idea to just pull the front diff cover off and inspect the gears to make sure there isn't any damage as well, then put int eh proper amount of gear oil.