Members have posted from time to time about their desire for a front diff LSD.
These discussions always end with "Some one should build one and make it available".
There is one that has been in production for a decade. There over 500 in the field and at least one has over 100,000 miles on it.
The reason you haven't heard about it is it has been marketed for GM fwd drag racing in Grand Prix and etc.
The company is Engineered Performance. The link is:
The owner posts as PontiacJeff in GM sites.
He's a good businessman and a gentleman. Hope you enjoy getting to know him as I have.