1998 Jeep Grand Cherokee / Differential

I am having issues with my Jeep rear transfer case overheating, it actually got so hot that it melted my fuel line and cought fire and had to be put out by the Fire Dept. At the time I didn't know what caused the Fire, so I replace the melted gas tank and fuel pump, all brake lines and fuel lines and rear transfer case oil and gasket. I have it running again but the rear Transfer case is getting very hot again and leads me to believe this was the initial problem, just wondering what I need to change to fix this overheat problem.