If the rear cover has never been off. There should be a tag bolted to the cover around the 1:00 position when looking at the axle cover. It will say limited slip fluid needed in the axle.
The other method is to take the numbers off the axle housing and cross them out on the Dana website.
The other way is to pull the cover and look.
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93 W350 w/duals, fuel pin turned, 6X.018" injectors, 4" exhaust, 3" air intake horn, Super HX40/16, auto, 3:07's axles, 2" skyjacker lift, B&W flatbed