Weird (i think tranny) noise,under truck louder

Hi Guys, Hope someone can please help or assist.
21015 taco v6 pre-runner sr5 with (tow package)approx. 100,000,miles from new 1 owner.
Just drove it across country and got oil changed before(Florida)then in California for return trip.
Correct oil and weight and ran flawlessly the whole trip, absolutely perfect, never missed a beat.
About a week ago it started to make a very light rubbing noise, almost sounded like the ac fan was on the next higher setting, its not the fan as i turned it off, but that's the type of noise.

I changed the oil again as it was 5,000 miles and hoped it would quieter down but it remained.
Oil was about 3/4 of a quart low???. always a new filter changed at 5k (5-30 semi synth after 60k, and on ).
No warning lights or anything, just a regular change ,I'm 57 yr old and this truck has never been over 3k revs always driven like its my last one.
Its most noticeable when taking your foot off the gas pedal, and letting it run on without gas.
I missed the tranny oil change as i thought i read somewhere that it was sealed, I've had 3 tacos in the past and never changed the trans fluid ever and got over 200000 on all (that's a lame excuse i know but it was what happened).
Any one any ideas ,im a pretty good parts changer but not so good on trannys, plenty of motorcycle experience etc.
Its only just a few degrees cooler than Hades ,in my my garage at the moment(Florida),so will do what ever is necessary if i have too, can also take a video on the i-phone if that will help.
The dealer wants $150 just to listen to it.
$300 for the fluid change.
Any ideas cheers in advance, Chris.

 

The transmission fluid absolutely can and should be changed.
Have you ever changed the differential gear oil?

 

No m8,
Thanks for the reply,
Not from new, but the lube shop in California showed me a sample and said it was fine.
I can defo do that,but its the front noise im worried about.
Planning a trip to Colorado in 1 month and am working my way thru it until this weird noise started.
How long does the diff oil last,my bikes last a long time???.

 

If you don’t tow, it last longer.
But I’d recommend not waiting longer than 50k. Some say you should do it sooner, and I won’t argue that fact.

Sure some people go longer, like never change it, but $40 is cheap compared to a new differential.
All you’ll need is a 24mm socket.