Investigating a 1976 XJ S2: donor or back on the road

Dear all,
Today I took delivery of a 1976 4.2 XJ S2 with manual gearbox, and I need to decide what to do with it: use as a donor or hopefully restore it. I’ll try and find a body specialist to give me an offer to fix the rost damage, and I’m looking at the mechanics. A first question I’d like to put to the group: when I lift the rear and turn one wheel forward, the other moves along in the same direction. So it seems I have a limited slip differential that is probably not standard on this car. Can you help me determine what diff this is? I know it’s impossible to read: this is the diff’s bottom, but I can’t see any metal tags attached as I have seen on other XJs.
Thanks in advance for some shared wisdom.

on rear diff cover there should be two small metal tags. one that will read something something PL. the PL means Power Lock aka limited slip. the other will have a 2 rows of 2 digits. These digits will determine will tell you what the gear ratio is. come back with the numbers, and I can tell you what diff it is.

Oh, one more thing. You lucky dog! a proper 4spd XJ with a limited Diff. How cool is that!!!


Hi Mark,
Thanks for the kind words and advice. I’ll check again if I overlooked a tag and come back to you to get your input on the ratio.
You certainly have an impressive collection; to which degree did you work on the cars yourself?

I knew I had a set of tags kicking around. Not sure why these aren’t where they belong. Stuck on the rear end.

The 75 tag indicates the year it was installed.(A deduction - not 100% positive).
P-L well that means PowerLoc - aka Posi, Limited Slip.
the 43 over 13 indicates the gear ratio. this Diff has a gear ratio of 3.31 to 1
other options are
3.07 to 1 has a tag of 43 over 14
3.31 to 1 has a tag of 43 over 13
354 to 1 has a tag of 46 over 13

here’s a fun fact.
Since there is a train plug at the bottom, that tells me this diff is a Salisbury; but it could also be the Dana 44 derivative, which in 76 these started to hit the streets with the XJ-S. The proper way of telling is if there is the letter ‘S’ or ‘X’ adjacent to the Serial number. The Dana 44’s really became the norm with the onset of the the Series III XJ12 Saloons & as mentioned above the XJ-S. There are other determining features such as the output shaft flange shape and number of bolts holding the bits together, and the pinion gear mount to the drive line is, without looking. @Kirbert will set me straight if I’m wrong.

If the rear diff becomes a project it’s really important to know what you have. The parts, (other than the brake calipers, pads, and handbrake hardware) aren’t interchangeable. I did a video that briefly covers this. Then there’s the quandary of brake pipes. Metric or Not Metric? that is the question. Im not sure the British Leyland even knew. After reading through the service manuals & training guides - not only did they have trouble spelling and writing a complete sentence - they content was completely contradictory. Which I also have a video more or less providing a heads up on how to tell metric vs SAE, UNF, Imperial, what ever you want to call it. Basically not metric.