make sure the shock absorber is actually charged with nitrogen. Mine came empty. I run a 12 inch rear wheel and have never had any clearance issues. I bought mine with a Staggs mono frame though.
you just need to put a tool of some kind in there and then put your lips over it. if you get a horible taste and feel like you are going to sufficate then your good to go, if not, get it filled.
my friend use to have that shock and said it was way too soft and he had to charge it. i never ran one on my staggs swinger, i run a ishock with an 800 pound spring.