I have a 2006 mustang gt with 88k miles. I bought the car used at 81k miles and was wondering if its a good idea to have my rear end fluid changed.

 

With an unknown service history, I think it's a good idea to replace all fluids (rear axle oil, tranny fluid, engine oil, coolant, brake/clutch fluid, power steering fluid) and filters (intake air, engine oil, fuel, tranny fluid, cabin air). You'll need to add a bottle of friction modifier to the rear axle oil to ensure proper lubrication of the limited slip differential.