1999 Sienna frontend humming sound

[10 goals]

My '99 sienna has a frontend gear humming sound like reardrive cars and the rearend hums. While accelerating you can hear it, the faster go the sound progressively increases (whines). When you let off the gas a bit just to maintain your speed comes almost silent. As you let off the gas and the engine slows down the van, you can hear it again.
Also theres a clicking (almost like a knock) noise accompanying the hum. The van has 78K. I think it's the differential gears or bearings that are bad.
I jacked up the frontend and tested both side wheels spinning under power one at a time and could hear the whine and clicking coming from the underside of the engine compartment/trans.

Is the frontend gears/bearings shot? How do you check the diff oil? Does the diff use ATF instead of 90wt. lube? By the way the transmission performs good, no problems in that area. Can the front CV joints make this noise? Can anyone help? Thanks!