I can't really answer your question but here's some info that might help.
I swapped a 5.3 into a 4cyl 95 jeep. I didn't have the PCM reprogrammed since I am eventually going with aftermarket gauges. So the PCM is outputting what I believe is a 4 cyl signal. My tach reads just about double what the gm PCM is seeing. That would imply that the jeep tach expects to see a 2 cyl signal. But that doesn't really mesh with what your seeing.
Did you check novaks site? I thought there was something on there about the jeep tach.