The 4.6 is an updated version of the 1960's Buick 215ci engine. A very good engine with low RPM Torque values. The 03 version had some issues with the Oil pumps, check to see if it was under the TSB for that. Other than that it was and is a very good engine. I would go with the 04 only because it was the last year for the Old Disco Body Style and it came from the factory with the Center Differential Lock (CDL) where the 03 doesn't.
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