I read a nice edition of the journals of Lewis & Clark last year (highly recommend, by the way--it was a good deal more interesting and exciting than I'd even hoped for). One unusual thing I found was their constant reference to using some kind of intelligible sign language whenever they were in some area in which their translators were no good. They imply several times that the Native Americans of the whole continent seemed to largely share some kind of basic sign language for rudimentary communication.
A few basic internet sources do seem to make reference to it--(e.g., "Sign language was highly developed among the Plains Indians as a method of communicating between different tribes"), but I wondered if anyone had a source of more comprehensive info on it.
Yes and no. There was a common sign language used across Plains Indian tribes - in fact, it came into use specifically because of their different spoken languages. But it was not used broadly across all American Indian groups, only those in the area approximately from the Gulf of Mexico to Calgary. Plains Indian Sign Language (PISL) was not used by tribes outside that region, though they may have had other signed languages.
PISL has the most research available of any American Indian Sign Lanauge. The leading researcher in this area is Jeff Davis at the University of Tennessee, with a grant from the National Science Foundation. The website for this research is PISL Research.