When you write "retainer", is it something that prevents the nut from turning? And, is the nut turning anyway? Now, either the nut is worn or the retainer is turning too...that sound right?
Or, is the nut rusted to the bolt threads? If that's the case, try PB Blaster or another good quality penetrating oil. Might have to sacrifice the nut and bolt though and cut the nut or bolt head off.
BTW, you shouldn't have to pull the shields off the top of the deck. You should disconnect the idler spring (under the deck...make sure you have that mower well secured when working under it!!!) and replace the belt on the pulleys.
If you don't have the manual, you can download it as a .pdf from the Woods website.
I have the same RFM, by the way...mine is just painted green and sold by Deere (Frontier brand).