Here is a picture of the transfer case mud I mean oil. 120k miles.

Click image to enlarge

It's obviously not ideal to re-use but if you're stuck and surfaces are OK and the washer isn't scored or harden then it's not a real issue although no doubt some will think it's a huge taboo to do. Typically though, copper washers go hard after a time.
You can anneal a hard copper washer to make it softer so it seals like a new washer. To anneal it you heat the washer in a butane/propane torch until it turns a bright red colour, then let it cool off slowly. Sometimes I've even annealed new washers that have been sat around on store room shelves for ages and have gone hard.
Obviously check for leaks after a few miles whatever you do.

Thanks Weejock that makes sense why they say replace washer. I had though that maybe they were more like spark plug crush washers. Good info on restoring their pliability.

It's not ideal but you can flip the washer over if it's only been used once.

A new one is just hogwash, I've never bothered with a new one, been servicing cars for over 30 years, never had one leak... the only time they leak is when they are over tightened and damaged... Don't do them up to the ridiculous torque they specify either, it's usually far too high, just nip them up and then a tad more....  Pete
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2014 L405 Autobiography SDV8 4.4 Loire Blue Ebony interior
2011 L322 Vogue SE 4.4 TDV8 Baltic Blue. Parchment over Navy Interior. Sold
2012 L322 Autobiography 5.0 Supercharged Ipanema Sand, Jet Interior. Sold
2002 L322 Vogue 4.4 V8 Epson Green, Ivory over Aspen Interior (Fatty Offroader) Sold
-Click for Project Fatty off roader-

I'm not so sure, i mean i agree using a new one every time is probably over kill. But i have seen plenty of sump washers on minis that were leaking like hell because they'd clearly never been replaced.

I reckon you can probably get two or 3 uses out of them safely by flipping them over, but after that they start to get a bit thin. I think the only reason service procedures specify new ones, is they're written on the side of caution as the washer could have been used countless times already and it's not worth risking a leak for a 20p washer.

A parts guy who claims he can't get a part number without a vin is just someone who doesn't know what he's doing. I've just found IYF500010 for the transfer case in a bout 2 min, it's the same on all L322's.

Rear diff is RYF000050 and RYX000020 for pre-2006 model year. And after that it doesn't look like they have a sealing washer just a plug (which might have a rubber seal bonded into it like the gearbox bungs)
The front diff doesn't have seals you just apply sealant to the threads.

I've just looked them up here.. http://cat.lrparts.ru/

Thanks all for the input. I know I have reused engine oil drain washers in the past without issue but now I know why they tell you use a fresh one. My Jaguar/RR dealer is about as small as they get. Maybe 6 people run the entire place. Parts guy asked me if the 2004 HSE was a full sized RR. When I met him I got the impression he wasn't an enthusiasts. I don't know if this is a RR dealer standard practice but they don't keep trade-ins to sell used. Only sell loaner cars as used. This could partially explain why he didn't have a clue about older RR models.

Did the rear differential today. Came out as clear as the new stuff.