You can take a thick cable with clamps on...ie. a set of thick jumper cables.
Clamp one end to the negative post of the Battery and the other end onto the Starter Motor body.
If it now starts fine then definitely bad grounding.
Put Voltmeter across the battery.
With engine OFF the voltage should be about 12.5 volts.
While cranking voltage should not drop below about 9.5 volts.
While charging should be about 13.4 volts.
Make sure all connections on the Starter/Solenoid are on firmly.
Glow plugs working OK? How does it start when warm?
Could be a bad Solenoid contact although the battery has been replaced already.
The Negative lead on the battery goes straight down to chassis I believe then there is a cable grounding the Firewall to the engine I believe....the 2005 Jeep KJ Service Manual will show them
You sure you have a large capacity battery?
