Re: $500 to spend and can't decide.
None of the above.
Radar Detector - Just about useless in areas that use laser (LIDAR). You'd be better off with a jammer.
Axel Back Exhaust - Noise maker only, no HP gain...if anything you lose low-end torque. You own an SUV, not a race car.
Engine Tune - Depends...I hope you're not talking some garbage chip. A custom "Protune" on a dyno is the only worth while engine tune.
Personally I'd bank that $500 towards your next set of tires.
2014 Black GC Limited 4x4 3.6L / Mudflaps / Front Tow Hooks / Pedal Set / Lighted Door Sills / Slush Mats & Cargo Mat / Locking Lugnuts / Phillips Headlight & Fog Light Bulbs / Skid Plates (Front & Middle Only) / Clearbra / Tinted Front Windows (35%) / aFe Dry Drop-in Air Filter