When my 1997 ford explorer 4wd 181000 miles it makes a grinding noise when i let off gas when i gas it it quits what could this be it does it only when i coast like driving 55 or it started in reverse but it shifts magnificent in every gear thats why im puzzled please help!!

I have a 97 ford explorer sport 4x4. When i drive at a constant speed it's fine but when i let off the gas I hear a grinding noise. if i put in nutral as soon as i hear it the noise quits areif I shoot the gas to it it quits. What could it be?

Could be the motor mounts.
Try this:
Open the hood, with the emergency brake set, and your left foot FIRMLY on the brake pedal, put the car in gear, and give it gas while holding the brake.
Watch the motor, see if one side lifts up. If the grinding noise is only on acceleration, if the mount is broken free, the motor will sit fine at idle but under pressure, will lift up.
If this is the case, you can replace the mount in question or both.