If you have a 2021.5 V8 chassis affected by the recall for low axle fluid

[NXR]

You might want to be calling your Ford dealer and getting on their list. From iRV2:

"Remedy Available From Ford as of 5/10/2021

The Jayco dealer just called me and Ford has communicated the remedy for recall 21S17 (rear axle fluid underfill.) The recall remedy from Ford is that if a VIN has more than 5 miles on it that dealers should replace the bearings altogether, no checking fluid levels first, complete replacement. The Ford dealer confirmed and is checking on parts availability for the rig we "were" going to buy. Last week Thursday, when I spoke with the Ford dealer the recall was still in advance notice with anticipated remedy available in 3Q. So things changed between then and today.

The big next question is parts availability. It could be that parts are back ordered for 3 months and we don't want to cancel our plans this summer."

[TitanMike]

The Mfr of my Tow Vehicle had the same problem back in 2004. Replacing bearings wasn't the issue. the underfill "smoked" the side gears in the differential and when they disintegrated the entire differential was toast.

They filled mine to correct level but the magnetic plugs started to collect a lot of shavings (like a tablespoon each after a few hundred miles) so they replaced the entire rear axle. Still going strong.

Chances are the "Underfill" was more like a "No Fill.