Older 6.2 are known for injector pump issues and last maybe 100K ,iles
or so (give or take) before repalcement is needed. If it is not
starting now in warm weather the first thing to check is fuel pump and
injector pump and check for water in filter seperator too.
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TheSnoMan.com

Greetings,

Start with the basics (and the old 6.2L was about as basic as it gets -
naturally aspirated, indirect injected, mechanical fuel injection pump).

First, drain your fuel/water seperator and change the fuel filter.  You will
need to check to see if the transfer pump (the inline fuel pump that moves
fuel from the tank to the fuel injection pump) is working - and there's no
telling just where it was mounted on your jeep.  Test your glow plugs next
as just feeling the wires isn't a good enough gauge.  Only some may be
working, or there may not be enough juice to them to heat them up enough to
get the motor started.  Since you didn't mention it then I doubt that you
have a glowplug light on your dash.  The glowplug relay was also an issue
with this motor.

The DB2 fuel injection pump on this motor is all mechanical, which is good.
It's cheap, simple and easy to replace.  It was a reliable unit but like all
mechanical diesel fuel injection pumps was suseptible to accelerated wear
and other problems from bad fuel.  Sulfer in diesel fuel was used as a
lubricant much like lead in gasoline, and today's low and ultra-low sulfer
diesel fuels can cause problems with older motors that relied on it for
internal pump lubrication.  The best case scenario is all it would take is
new seals in the pump designed for the newer fuels; the worst case is a
complete replacement of the pump.

And finally, the majority of these light duty diesels came with two
batteries wired in parallel to provide enough amps to start the motor.
Diesels typically take much more juice than gassers because of the high draw
from the glow plugs and the need for a high torque starter to overcome the
high compression ratio in the cylinders.

Good luck - Jonathan