I joke a lot, and I can be pretty silly and random. Most of my humor is related to story-telling; when I talk to people about things that I've experienced, or that someone else has, I act out reactions, and make over-the-top facial expressions. Besides that... I dunno. I say things, and people laugh, and it's nice, even when it's unintentional (as long as it isn't embarassing).
I don't know many SJs, but all the ESTJs I know have funny bones, and the ISTJ I know cracks dry jokes once you get to know her. With both types, though, I think both types spend more time laughing at other people's jokes than making their own (in my experience).