I've had this happen several times, but all for different reasons. First one was faulty ABS sensor (wheel speed sensor, replaced under warranty). Second was loose connector for the solenoid of the differential. Third was low battery voltage. The voltage issue and the loose connector threw up the ELD unavailable error but didn't leave a diagnostic signature (the first registered as an ABS/ESP error code) so I'd get your battery checked and poke the diff wiring a bit before spending that much on a diagnostic. Also of note: a wonky ABS fuse can also cause an ELD error but won't actually blow the fuse until you trigger the ABS system, read below for more details.
The more difficult issues would be a weeping/leaking differential, especially around the seam of the differential case/housing. That would be a big issue, and might be a deal breaker as Fiat only sells the differential as a whole unit (though they do sell the differential case gasket separately now, finally). Then again, they also used to claim the diff to be non-servicable and called it a lifetime part but changed their minds around 2015-2016 since while the diff itself may last the life of the vehicle, the differential lubricant has a service end-date which means you might just need a fluid change.
Long story short, start with the simpler solutions and work your way up. Check the battery voltage and the manufacture date, making sure the battery itself is in good condition (not leaking, no bulges anywhere) and if it seems good but the voltage is low, take a long drive up the motorway and get that alternator pumping juice back into the battery (also good for the diesel particulate filter regeneration if it's a 1.3 MJT). If that's not it try a few emergency stops and see if triggering the ABS throws up an error. Check the fuses, replace any that are obviously blown, or have stress marks around the plastic indicating heat cycles (discoloration and banding in the plastic surrounds) that haven't yet triggered the fuse to pop. Check the wiring under the car, for the diff and the wheel speed sensors. If you can get the car up in the air on jacks, see if the center diff engages automatically at low speeds and disengages above 50 kph indicated speed.
Last but not least, find a cheaper dealer (or haggle), or an independent specialist who knows their Panda 4x4s and has
MultiECUScan or an original Fiat diagnostics setup to properly parse the codes and diagnose the issue.
