How can I tell what size differntial I have under my truck? I haven't bought the shop manual yet, but understand that there are three different types. It's a 2001 F150 regular cab with 3.55 open diff. It looks like the 8.8 under my Mustang but the pinion snout appears to be longer?

Thanks,

 

4.2=8.8
4.6=8.8
5.4=9.75
F150HD/250 5.4=10.25
and I think the V10 and Power Stroke use the 10.25 also, somebody correct me if I am wrong on the V10 and the PS

LHM

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2000 F-150 Super Cab XLT 4x4 ORP 5.4liter V8, Towing Package, Rear Slider, Remote keyless Entry,3.55 LS, Black Exterior, Medium Graphite interior, manual shift 4x4. Current mods Gooseneck hitch,Ford Custom Molded Splash Guards, Muth Signal Mirrors, SUPERCHIPS Performance Chip, Line-X Bedliner, STROBES!!and Y pipe with duals.