My wife and I have been driving our gifted Cherokee much more lately after getting the brakes figured out. We both noticed at 40+ MPH if you let off the gas you hear a whining/whurring noise under the car. It sounds like it's from the front but it is hard to tell. I thought it might be a wheel bearing from searching the forums but the noise is only apparent when you let off the gas. I switched the transmission into neutral at one point and the noise did not go away.

Would this be the U-Joint or an issue with the differential? Would there be an easy way to tell with the car jacked up?

I inspected the U-Joints and there is no play in them but they both look old and a little rusty and no zerk to add grease. The differential fluid was just replaced because the gasket was leaking.