I've got a 1998 E350 extended club wagon SRW, Limited Slip, 3.55. The rear end is making noise at highway speed. Not really a howl, more like a wining sound...like a hydraulic motor would make.

The axle code on the door tag is "C9" which I read stands for Ford 10.25 Limited Slip. But it doesn't have a Ford axle, it's a 10 bolt Dana. The tag on the diff cover reads F8UA. Is this a Dana 80?

 

That's a Dana 60. You can search the BOM (606049-2) here:

Dana Holding Corporation - The Expert Axle Bill of Material

Here is a page with all parts specific to your axle for rebuilds and such

Axle Builder | Dana 60 Rear Low Carrier 32 Spline Axle Parts

Unfortunately you have a 32 spline axle so aftermarket support is limited.

 

Awesome, thanks for the info.