I have a thread up with testimonials on our website, can I: A - Use their nicknames + testimonial on print material? B - Use their First/Last name + testimonial on print material? C - Use their First & Last Initial + testimonial on print material? All without their consent?
Exactly, you can do whatever they agreed to when clicking the "I agree to your terms" box when they posted the comment. What? You didn't have one? Then I'd be careful.
As others have stated, you should include a clause in your TOS about the content placed on your website. If you clearly acknowledge that your retain full rights to all content placed on your forum, then you're in the clear. Since you have a thread specifically for testimonials, you should make a notice within that thread of your intentions. If anyone has a problem with it, they can just choose not to submit anything, and it doesn't do you any harm.