What wire should be attached to the ground terminal on the starter relay? Currently it has a black/white wire coming out of the main harness.

There are dark green wires attached to both the ignition and solenoid posts. The solenoid post wire tees into a green/white wire coming out of the harness.

 

black/white should be correct for ground at the relay

 

I went out and hooked up a wire from the ground post to the neg terminal on the battery and it turned over with the key. Any reason why the black/white wire wouldn't be getting ground? Is there any adverse effects to running a separate ground wire and leaving the factory one off?

 

It's grounded through the Transmission Range Switch (aka the Neutral Safety Switch). This is a VERY common failure on Cherokees.

Luckily, you don't have to replace anything, the neutral safety switch just needs to get a good cleaning.

Follow this writeup and you'll be back on the road in no time:
http://bc4x4.com/tech/2010/jeep-neut...ty-switch-nss/

 

Thank you, sir. Looks like another little project to work on.