for the past week my trucks been making a loud humming sound when I let off the gas at 40mph from the rear of the truck if i got slower it wont do it.My tires are only 2 months old they are fine.The rear axle fluid is full I cant seem to find whats making this sound anyone?

 

Where is the sound coming from? We can only guess unless you provide some more details......

 

Does the truck have a lift? How are the ujoints? Carrier bearing if it has one?

 

No lift stock truck stock tires sounds like a tire sound but like I said my truck has stock tires that are 2 months old seems to be coming from the rear axle the sound and today when i shifted from park to drive i get a loud clunk not sure what thats all about makes the loud clunk going ito reverse also so now its making 2 sounds for some reason.

 

All of my money is on c-clip failure in the rear diff, with a possible side of loose pinion nut.

 

https://www.youtube.com/shorts/KmG2yUcLVjI

 

That sounds like a driveline noise. Since it is occurring on deceleration it's likely you need to adjust pinion nut preload.
Other possibilities are wheel bearing(s), front pinion bearing, or u-joint.
Did the noise start recently, or have you noticed it for a while? Did you have an event where differential received a lot of torque suddenly, with rear end hooking up after spinning on an unprepared surface?
Let us know if you want info on adjusting the pinion nut. Not a difficult task, but there are a lot of steps and a few tools required.