I replaced the battery, now the truck wont start.how do i reset the security system?

Video help

https://www.google.com/search?q=1996+jeep+grand+cherokee+security+reset

..

Passlock Reset procedure Reset security system on chevy buick oldsmobile...

If the security system has been reset ( no light on/flashing ) you may have a neutral safety switch problem, Will the vehicle crank over in neutral? In most cases, the neutral switch grounds the starter relay and allows it to operate. Does the starter relay connector have 2 powers? One constant and one with key on/cranking? If these power supplies are there, suspect the ground side of the relay to be the problem and possible neutral safety switch

Many reasons why a Cadillac security system activates, and the procedure to reset it can get very complicated. You can try disconnecting the battery (negative cable only) for a couple of minutes and then reconnect to see if the problem goes away. If not, you will need to have a GM dealer reset it.

try using a scanner 9code reader to reset the CPU program as it sound like the program is corrupted from the battery change.

If the car does not have a security system, I would check for voltage at the starter from the battery and from the key switch.
The power from the key switch goes thru the neutral safety switch before getting to the starter.

try disconnecting the battery should reset the system

CHECK for loose wires, battery cable is clean, install a new starter.

The security system (VATS/PassKey) is having problems, it's typical on those cars. You can find a great write-up here: http://newrockies.ca , it was the only thing that saved my Park Avenue.