Re: Speedo and Odo at 50%, 4x4 lights stuck on
In 2001, the sensor on the top of the rear diff is for the ABS only. The 2001 raw speedometer signal comes from the OSS sensor in the transmission case (automatic) or in the extension housing (manual). The OSS could possibly be bad but doesn't often fail.
Actually, I think that your problem is not in the speedometer system at all. The speedometer is corrected by a multiplier in the PCM whenever the 4x4 Low indicator light is on. It makes the speedometer read lower to correct for the 2.48:1 gear reduction in the transfer case. I believe that whatever is causing that 4Low light to be on incorrectly is also causing the speedometer error. Get the electronic 4WD fixed and the speedometer problem should go away too.
2003 FX4 Level II, Supercharged 5.0L V8, Headers, Duals
BW4406 manual transfer case, Aussie Locker, Torsen L/S, 4.10's
Bilstein 7100 rezzies, Cadillac/Mustang rear discs, Duff traction bars
Keypad Entry, Lock LEDs, ABS switch, sway bar discos
Heated seats, Explorer EATC, Trip Computer and consoles
2003 XLT 2.3L M5OD-R1 reg cab beater, 2011 Taurus SEL, 2016 C-MAX Energi Plug-in Hybrid