Hope you have better luck than I did, don't think you're going to find a 38mm tire that works in mud. Lot's of 'gravel-grinder' suitable tires in that size, but none are mudders.
However, if you can work a 45mm into your frame, Panaracer makes a 700x45 tire that is a mud boggin' fool.
I have a new pair of Continental Twister Pro 700 x 42, They were Continental's race tire but now have been replaced with a wider tire. like most Continental tire I don't think they are a true 42mm, I bet they are closer to 38mm. If you are interested, let me know.
I was looking for the same thing, and finally I found a set of Maxxis "Mountain Cross" in x38 on ebay, they were NOS, and are really aggressive for a cross tire, treadwise. I really like them.
Beth, another thought, there are a good number of narrow 29" mountain tires out there, maybe your frame has clearance for them?
Another NOS dig is the old IRC V-Cross. I found one at an old shop during my travels and it works well for me as a mud tire. It's listed as 700 x 38 but it may be a tick narrower. Could be rough trying to find a set of those.
I have a set of Panaracer FireCross tires and can say I agree with Scott's statement, but they are not near a 38. More like a narrow verson of the FireXC tire. BIG knobs for a 'cross tire.
Beth - if you'd like, I'd send you the tire. I only have one but you can PM me if you want and it's yours.