A diff only ever drives one wheel at a time.
They are designed to work that way
So only one driving at the rear is normal.
If you hold that wheel the other should rotate.
If not then check the drive keys in the axel or for a broken / stripped axel.
Not sure how the front works on that model but again a diff will only actively drive one wheel although both should turn.
It all depends upon how the front is powered.
Simplicity service manuals are a free download from the Simplicity web page
From memory it is "simplicitymanufacturing" but others in particularly Mark should be able to steer you much better.
His web browsing skill are much better than mine.