Jeromeo,
The way I described it earlier is correct, so what you are experiencing is also right.
with the diff unlocked, only one wheel drives. From memory, the drive wheel is the one on the
opposite side to the diff lock.
If that drive wheel is on say wet grass, it is possible for it to slip and there for you get what you are experiencing.
When the diff is locked both wheels drive at the same speed.

Here is a section from a greenfield catalogue explaining about the diff.
[Linked Image]

I have actually pulled one of these diffs apart and reassembled, so I speak with some degree of knowledge.

Have you checked the air pressure in your rear tyres? Maybe the are over inflated?

I hope this helps.

Cheers, Murray