Use the bleeder screw, that's what it's there for. Chances are you had an air pocket in the core, and as the air pocket heated up it created enough pressure to crack one of the solder welds on your core. Hope your mechanic buddy is willing to help you in the event you damaged the headgasket and/or drop a valve seat since you've overheated the engine.
That heater core looks to be copper, not aluminum. It's also possible that you may have tweaked one of the hard lines coming out of the core when reinstalling. That's the one issue with copper cores, they don't do so well with slight bending as the solder joints can crack easily.
'00 WJ 4.7 Limited- Some stuff
'03 WJ 4.7 Laredo- Some more stuff
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It is what Jeeples has *PARTIALLY* stated.