Drivetrain whine like it's going out of style

[clean2g]

I have this whine coming from somewhere in the drivetrain (mid to rear of the car). It has progressively gotten louder since last year. It only whines when I let off the gas, and seems to be loudest when I let off the gas at higher RPM's. I swear it's coming from somewhere mid to rear of the car. I doubt it's the tranny, it was rebuilt by Shep about 40k miles ago. I'm thinking t-case but I'm not sure.

The t-case was leaking a little fluid at one point but that stopped. Fluid level was fine and I topped it off just in case a while back. I had actually taken it to the dealer to see if I qualified for the t-case recall about a year or two ago when I noticed some drips from the t-case on the garage floor, but they said doesn't qualify and wouldn't budge. I don't want the rear end or the t-case to lock up on my while driving.

Has anyone dealt with this? Any simple checks I can do before I have to actually remove and inspect the t-case and/or rear end?