tranny problem with 90 Bonneville with 3800 V-6

My sister is driving a 90 Bonneville that she claimed was running
roughly.  But she wasn't too specific about what it was doing.

So I went out to see what was wrong - the car started up fine, and I
backed out of the driveway, went to put it in drive and it wouldn't move
forward other than to coast downhill.  It won't go forward in any gear.
 SO I had to back it back into the driveway.

I got it up on jackstands, and the left front wheel will turn when put
into drive or other forward gears, but the right wheel won't.

So I was wondering if anyone has any idea about what that might be?
Differential that needs to be serviced out of the car, or something
simple that could be fixed by dropping the pan or making some other
adjustment?