I have two badly worn tires on the rear axle of my 2006 Honda CRV and the two front are in good shape. They were all purchased new and have been rotated at from 6,000 to 9,000 miles. They now have 22,000 miles and the rear axle tires are shot. They are Michellin 70,000 tread mile tires. Do I need to buy all four tires $750+/- or will two do? Why are these tires wearing so unreasonably fast?

That sounds like you rotate frequently enough. Tire pressures checked frequently, too? Alignment checked at least annually?
I'd like you to define what you mean by "badly worn" when referring to the two worn tires.
Are they worn unevenly or are they worn prematurely?
Normal tire wear across the entire tread face all the way around the tire's tread at such low mileage could be a warrantable claim.
You'll have to contact the tire shop or manufacturer or both. If you have a legitimate claim, you'll still have a journey since they don't like to be wrong.
If however the tires are worn unevenly across the tread face or it's uneven around the tire, that may indicate other problems.
The only choice is to have "them" look at the tires and make a decision. Keep your records and don't back off. Good luck.

Tires are worn down to the wear bars without noticeable cupping. Does this model Honda have an all wheel drive which requires the absolute same size tire on both axles? I would say the front tires are one half or slightly more of the original tread depth still remaining so there should be quite a few miles left. Buying four new at this time seems unreasonable to me.

Oh, you can buy just two.
Take a good look at the tire sidewall and write down the size. Then take a look at the placard on your vehicle's door jam to make certain you're installing the proper size.

I have a 2006 and purchased a 2010 and now having the same problem on this car. They won't tell you. The problem is the rear axle is to short! They have to install a larger one! Fight for it. It's a car problem... not tires or rotation ! There is a class action lawsuit going on in Ca & NJ regarding this matter.