Hissing noise while coasting

I apologize if this has been asked before or if I am posting in the wrong section but I'm at my wits end and could use a little help
I noticed a strange hissing sound while coasting in drive that goes away when I shift to neutral and remains while 4wd is active regardless of gear after I drive for 10 or so minutes and get everything warmed up-rear diff lock also wont engage, I had a 2000 mile road trip when I noticed it and I don't recall hearing it before
I've replaced my transmission fluid, front and rear diff fluid and transfer case fluid about 10k miles ago
I own a 2011 Tacoma TRD TX
I appreciate any feedback or any info on the correct place to post if this isn't it
Thanks heaps

 

I thought diff lock engage was in 4LO only?

 

It might just be, I've only ever used it when it's actually useful and I just learned that it needs to be turned on for a few miles every month to keep it working correctly