I have a 2006 4.0L and I put a flowmaster 40 series on it and it sounds great! Mine has 2 catalytic converters on it so that already muffles it down quite a bit. Also right behind the second cat it bells down to a smaller pipe so i took the bell off and ran the same size pipe back to a single in double out muffler with the duals out the rear. Sounds great and wouldn't have it any other way. Its quiet inside the cab but sounds mean when accelerating. Once you hit cruising speeds on the highway you wont hear a thing in the cab

i had a glass pack installed at my local muffler shop for $83. sounds great but not too loud

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2004 Step-side xlt4x4
4.0L 5-speed
33-12.5-15 BFG MT km2, tbar crank,Projected HID's, blacked out tails, LED '06 corners, LED tails, LED back up lights
Intake, Chip, MAF, TBS, Straight Piped, Air Lift with onboard air system
Black interior with silver accents, LED Gauges, '04 Fx4 buckets, '94 explorer shorty console, '03 Fx4 ll shifter arm w/hurst T-handle

Strait pipes are raspy...especially on a V6...