Regular cab howling when letting off throttle

Hey guys.

I've noticed that I get a howling sound when coasting at around 55mph. When I get back on the throttle it goes away, only be when coasting. Seems to recently be getting more noticable but I did just swap to some winter M&S tires.. Don't think its that though. 160k, Manual Trans. Doesn't seem to be coming from the rear end but more like transmission/underneath me. I did an oil change the other day and everything felt right and tight. U joints are greased. Any thoughts on where to start poking around?

Edit: I do have an add a leaf in the rear with 3rd gen swap suspension.

 

That would be the infamous RC slip joint noise. It happens to lifted reg cabs and even some stock ones, but it's just a nuisance.

Some have had luck flipping the shaft around, or adding a zerk to the slip joint and greasing often. Guaranteed fix's are a aftermarket drive shaft, or drive shaft spacer from 4crawler.

There are a lot of threads on the issue