Okay, it will work for you if you follow these steps:
1. Copy the BBCode for your GIF.
2. Make a post in the Monster Truck GIFS thread, paste that BBCode as plain text in that post. (This will post the GIF to the forums ONLY, it doesn't work for your signature.)
3. Copy the image URL from the GIF you posted.
4. Now, when making your signature, act like you are posting a picture, but use the GIF's URL! BAM!