1992 Ford F-150 Rear end noise

I have a 1992 for F-150 with a limited slip rear end. It has been making wierd noises in the rear end. It starts out as just a slight clicking noise but some times it makes kind of a grinding noise. The noise goes away when I am in consistant movement of about 30 mph or faster. If I really baby it there is hardly any noise; if I give it a good amount of throttle is gets really loud. If anybody has any words of wisdom I would really appriciate it.