There's always other races - might not feel like it now but it's better to skip it and recover quicker and stronger than complete the race in a poor time (or even worse have to drop out half way through) because you're injured to start. You risk doing more serious damage that could put you out of racing for a long time.
I was supposed to be doing the Grizzly this weekend but have had to reluctantly step down, so I really know how you're feeling. I have been really looking forward to it but realise that it's better to just join the support team for this year and be able to recover from my current injury quicker and continue racing throughout the year instead of potentially injuring myself for a lot longer.
In either case I'd advise trying to get a sports massage to help release all the muscle tension. If you've not had one before they do absolute wonders. You might be able to get a last minute one today and the therapist will be able to give you better advice as to whether you are in a position to compete or not. My feeling is always if you have to ask you're probably not fit to run - but that's just me 
Best of luck and I hope you're back to form quickly!
