Oh well, your first post is misleading or I can't read.

You can check your t case snap ring on the output shaft by droppping your drive shaft and removing the four bolts on the extension housing. And pulling that off.

Dont just check to see if it's there. Make sure it's snug and there's no play.

If it's missing, the damage is likely already done.

 

I might try that, might as well if I'm going to drain and refill the case

 

Like I said this only happens when I let off the gas, while driving normally in 4wheel I still have to let off the gas. The noise is just a lot louder. If something we're rubbing or scraping the noise would be constant wouldn't it? Also I remember from when I bought the truck the transmission mount was broken a little, is it possible a fully broken transmission mount could cause this?

 

What do you mean by "When driving in 4wd normally"? Where are you driving in 4wd? What type of surface?

The whine is when you let off the gas right? Can you tell about where it is coming from? The rear of the truck or the middle?

 

This sounds like a u-joint issue to me. Set the parking brake and check for play in the drivelines. Might also try pulling your front drive line, even if it feels good, to see if it goes away.
In my experience, a u-joint getting ready to go will stay silent (relatively speaking) while under load, but start chattering when coasting or compression braking.
Hope its a quick, cheap fix for you.

 

Honestly I forget where I was going with driving 4wheel normally...haha just not beating it and such. There's no play in my rear drive shaft, is the front spin constantly also? Like I said it still does it in. Two wheel,just worse in 4wheel

 

Under perfect conditions the front drive line doesn't move. Of all the 2nd gen dodges I've driven and been around, I can only remember 1 that didn't. Most of the trucks I've owned or driven as work trucks, the front drive line was stiff to turn when in 2 wheel drive. The others had the lock collar in the cad engaged all the time because of vacuum issues, making the drive line spin all the time.
As for the broken tranny mount, it might cause some issues. It could allow things to twist wrong when you let off the gas.
Another quick test... Speed up and slow down in reverse. Does it behave the same speeding up in reverse as when slowing down going forward, and vice a versa? This might tell you if the noise is torque related, which could lead to tranny and/or motor mounts.

 

It does do the same in reverse, I do have a vacuum problem though,occasionally the 4wd light will come on when driving,but hadn't been a problem like this. I'll try replacing the tranny mount tomorrow, motor mounts are fine to my knowledge. When I rev from under the hood the whole truck torques and the engine stays stationary

 

Okay so my front drive shaft turns a quarter turn, not because of the u joints. It turns in the differential and transfer case, is this normal?

 

Pretty much. It's a bit excessive, but, on a high mileage truck, it isn't really a surprise.

Can you make it make the noise with the rear on jackstands?