if theres any chance that its not rocker noise , but a light piston slap, its normal for 4.0's to have a little slap at idle- low rpm. well, not "normal", but very common, and not a big deal, according to my local chrysler mechanic.
if you can, thread in a actual oil pressure sending unit and gauge into the port on the engine, so you know exactly what presssure its putting out at differant rpms. my $0.02