My cover rusted out... Have 9.75 12 bolt Limited Slip rear. What gear oil do you recommend? I'd prefer synthetic. Do I have to use the ford friction modifier or is another brand ok? Like lubeguard?

 

Also I noticed on top there was an electrical plug on this diff what does it do? Do I have a e-locker I'm not aware of?

 

75w140 is the gear oil you use and if you have limited slip use 2 bottles of friction modifier. The electrical connector on the top front of the diff is for your speed sensor. If it comes in from the driver side and is near the rear cover and above the axle tube you have an electric locking diff. Does your 4x4 switch pull out? If it does then you have an electronic diff. If it's electric don't use friction modifier

75w140 is synthetic and I would go with Ford friction modifier, it's probably the best on the market (I work at Dodge and still prefer it) and it's only about $10/bottle