Could it be a heatshield or something else exhaust-related? Everything has a resonate frequency at which it vibrates, so a loose part might rattle at 900 rpm (when idling in gear) but stop rattling at 1000 rpm when idling in neutral.
Kinda hard to troubleshoot without being there to hear it. Maybe get the wife to hold the brake in gear while you poke your head around under the hood, behind the glovebox, underneath...
So I installed the flux capacitor but the Jeep just won't do 88 mph.
-'99 WJ Limited, 4.7, Patriot Blue, 2" BB, 31" Mall-Terrains, Pavement Princess
-'97 TJ, Stone White, 4" Superlift, 33" SSRs, lunchboxed rear, 12K Titan winch, and 2.5 litres of exhillirating performance.