Lice are tan to reddish tan and do not cause diarrhea. They are found usually around the skin in the vent area where it's very moist. They can be seen easily moving about. But lice only munch on skin and feather dander, so they only cause health issues when they're very numerous.
Yellow runny poop most often points to reproductive infection. This can happen to multiple hens at once if they are in a poop infested environment. The bacteria can enter through the vent and work its way up the reproductive tract. Extreme lethargy indicates the need for an oral antibiotic, even though it may be late and not be immediately effective, but still worth a try. What antibiotics do you have on hand? Are you in the US ?