Got one on my AMX. I bought it off eBay a couple of years ago, think I paid around $60. Put it on with some stainless Allen bolts, looks great and won't rust.
I haven't seen them advertised in a while. You might check with companies that sell Jeep parts, since they used a 20 from 76-86.
Quick google search:
http://www.oconeeoffroad.com/servlet/the-3108/Differential-Cover,-Rear-AMC-dsh-20,/Detail
Bob
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