The part u are talking about is the speedo sensor.Your leak will be coming from there. Send your postal address to Reg at Backdraft and he will send u a new "o" ring. It is just a a push in sensor. The "o"ring holds it in. U fill the
oil untill it is level with the top plug. Your diff is an e36 diff used in3 series from +-1993 year model. Yours is probably out of a 325 or a 328. Do not overtighten the rear cover. It is unlikly that it will leak.