Noise at speed, not wheel bearing

[DND]

I started getting the customary kind of noise on the freeway that indicates a failing wheel bearing except when I jacked up the car and spun the wheels to figure out which one I didn't find what I expected.
It seems that both of my rear wheels are hard(er) to turn (no my parking break was not on).
Have I maybe got something wrong with the rear diff?
Might I be so lucky as to just need to add 75W90 oil to it?
Other thoughts?

[DND]

I'm surprised no one had any thoughts to offer.
I took it into a shop and the oil levels were all good, differential seemed fine to them, but they think my transmission needs to be rebuilt.
Anyone got any links to doing a 6 speed swap? Or know of any good condition 5 speed transmissions that are inexpensive?