I, and others, have posted a lot of info on Transit drivetrain options on the usafordtransit forum. Yes the rear diff is a standard Ford Stirling. Rather than a clutch pack Ford LSD, most would agree the geared TrueTrac LSD is a better choice...I have one F and R in my Quadvan 4x4. Rear was $1100 installed i think at a local 4WD shop. I toyed with the idea of an ARB locker but IMO that only makes sense if you’re doing a 4” lift, low roof van build with >32” tires.