installing an aftermarket tach
 

would anyone know what wire is the ground side of the dist. i have a 93-96 3.9

The wiring diagrams are in the FAQ in this section, just a few posts above this one. That will give you which side of the coil is the ground which is from the PCM.

It might be easier to use the Tach wire that is for the gauges, but I don't know if you will be able to use the same signal the PCM gives to because the Tach uses a module to run the tach, which I don't know if the aftermarket tach will use a similar module with the same type of signal input. The Tachometer on the gauge has the module on the back. The PCM provides the signal at one of the pins. The gauge plug will have it on it, check the wiring diagrams for which one it is.

Why wouldn't you be able to? That would be the best place to take the signal from.
Btw, you are looking for the ground side of the coil not the distributor.