You are experiencing a high voltage drop under load. Most likely cause (as mentioned above) is a broken or corroded battery cable and/or connections at the battery, starter, grounds, or starter relay.
If your cables are original to the Jeep, toss then, they are more than 20 years old and should be replaced. They break down internally due to corrosion of both the cabling itself and where the connectors attach to the copper cables.
Carter BBD feeding 4.2 Nuttered 6 banger = 22 MPG Hwy
If it was easy, everyone would be doing it.