I can get Amsoil Severe Gear 75W-140 cheaper than I can Royal Purple right now.
Amsoil sells a limited slip / locker additive and they do say you shouldnt need it when using their synthetics. However, they do put a disclaimer that in some instances you can get chatter but you can just use their Slip Lock additive.
I am using RP 75W-90 in the front diff and RP Syncromax in the transmission & transfer case but only because I already had it lying around.
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