Not to sound completely helpless but I cannot manage to unhook the wire coupler on the rear sensors. Fronts came right off. There seems to be a little snap thing one Both sides. Three hands also seem to be required

 

There is only one tab to press, but it can be a PITA especially on the passenger side. Take a look at the pic, press the tab back and pull down on the connector. Go easy because the plastic gets brittle with age.

Before you install the new sensors putting a little grease on the outside of the connectors and index makes getting things apart easier next time. Good Luck!
Not the best video, but it give you the basics (skip to 2:45) > Oxygen Sensor Replacement For Discovery 2

 

Do you see any problem with soaking the connectors with pb blaster or wd40 to loosen them up?

 

That's fine, I would and have used WD-40. It will help and you won't need much.