Personally, if I pair of Mavic Kysrium's on the bike originally, and the front one had held up well for 20,000 miles...I'd buy a new Mavic Ksyrium rear wheel (well, I'd get the rear wheel checked first like other people mentioned, but assuming it was broken and couldn't be fixed).
Not only have you proven that the wheel is durable, you might be able to get one that matches the front wheel and avoid paying to replace the front wheel. :-D