I've just had my 2008 FLHR Road King rear brake totally fail due to an ABS problem.

Are you sure it's an ABS problem?

Heck Boozo. Don't lose heart. Sorry to hear but the bike is 10 years old now. Lot of water under the bridge since that bike was built. May be able to get a second hand part.

You dont hear of many abs module fail ... But any component can fail I suppose .... I take it , you took your bike to a dealer to confirm the bad news with the use of their Digital Tech workshop tool  .....  Did the dealer by any chance tell you of the error codes that may have come up regarding faults in the abs system ????  And was you abs dash light on solid when the rear brake became rock hard ????  Not really trying to double guess the dealer, but they  have a habit of being part changers, specially with electrical and abs problems ...  As you are talking about a fair sum of money ,, I would like to be confident that all checks have been done,, and if it comes back to the module, then so be it ......   

As an aside, I have had 3 ABS / TRACS controller and 2 unrelated MAF failures in my Volvo Wagon in 300K Kms. Should have been a safety recall (according to forums) but at approx $500 for the part (refurbished) each time (not inc labour), it's good business for the mechanic. I bitch and complain but in the end, I like the car, pay the money and move on. Having a good relationship with the mechanic helps.

Boozo ,,, Been reading on the internet that the Harley abs system, (module) , has been known to fail , and have similar symptoms as you experienced,, (solenoids lock closed, not allowing fluid through , so not allowing the brakes to default to normal braking, without abs), .... Can be rear or front brake that goes solid , resulting in no brake.. Bad enough one brake going,, but it would  be catastrophic if both brakes failed ...  Seems to be 2008 --2010 models .....
When you get it all sorted , post up and let us know the results ....  Cheers ....

I'd be considering removing the abs completely for those $$$