When you replace the freeze plugs get brass, not steel. If you have a 4.0, it has five. Another thing to check is the thermostat housing. I've heard they typically corrode from the inside. Mine did. A new one is less than $30. While you're at it, install a new thermostat. Stock is 195 degrees, but 180 is a good option. I installed a Stant Super Stat, but Honeywell also make a good one. Don't buy a cheap one. The good ones are inexpensive enough. Change all of the hoses too. How's your water pump? If it's questionable, consider a FlowKooler high-flow pump. Again, if you have a 4.0, buy a new heater hose outlet tube from the dealer. This screws into the water pump.
Here's what I'd do the next time I change my coolant: After draining it COMPLETELY, put in a 16 oz bottle of Royal Purple Purple Ice Radiator Super Coolant. Then mix your antifreeze with DISTILLED WATER. This will eliminate any minerals you might have in your local water supply. Then fill your system with the 50/50 mixture.
'96 ZJ, 4.0L, 42RE, NP242, stock D30 and D35-LS w/ 3.73, 4-1/2" lift, 32" BFG ATs, Magnaflow, JKS track bars and disconnects, spare tire carrier, Optima red top, ROLA roof rack, KC SlimLite driving lights, CB and Yaesu ham radios, GPS