Can I adjust the idle or gears on a 2004 Ford Mustang?

The idle isn't adjustable as it's controlled by the ECU. At any rate that wouldn't solve your problem, as you'd still be turning 700 RPM in 3rd gear. The RPM you're turning at a given speed is determined by the ratio of the gear your in, the axle ratio, and tire diameter. Idle speed doesn't factor into it. 

You could swap out your rear end gears, but I don't think that's what's going on here.  You have 3.27's in there now.  Assuming stock sized tires (225/55/16 or just under 25 inches diameter) If you have the T-5 manual then you should be be turning just a little over 2500 RPM at 30 MPH in 2nd gear. But in third gear you should be about 1700 RPM at 30 MPH.  The only way you're turning 700 RPM at 30 MPH is if you're going from 2nd all the way to fifth. in 5th gear at 700 RPM you would be doing about 30 MPH per the math. 

It sounds an awful lot like you're going from 2nd gear to 5th gear when you're in town. 

"Can you"? Since you're asking the question, apparently not.

Should you? Unlikely. You'll be on a better trajectory if you learn to correctly use what you have.

There is no problem with the car. You're creating the problem by not using the car correctly.

What you wrote doesn't make sense. 

Adjusting the idle has nothing to do with anything.

Adjusting the gears is some impossible fantasy. 

See a Ford dealer and discuss the problem.

See what they say.

For fun, I decided to perform an experiment with my Ford Ranger that has the 4-cylinder engine, a 5-speed overdrive transmission, the factory 3.73:1 rear axle and the factory sized P225-70R14 tires. 

At 30 MPH my engine speeds are:

1st - 4980

2nd - 2940

3rd - 2030

4th - 1340

5th - 1060

You likely have a different axle ratio, probably 3.27:1, but it wouldn't explain 700 rpm in 3rd gear.  It should maybe be around 700 rpm in 5th gear.  So either your tach is highly inaccurate, or you are shifting from 2nd to 5th.

Transmission is likely shot.  Sell the car before you lose all your money.

Your engine isn't "struggling" at 2500rpm.  Leave it in 2nd at 30mph if it is indeed turning 2500rpm in 2nd at 30.  Don't lug the engine in 3rd to go 30 - that's harder on it.  I suspect your speedometer isn't especially accurate if at 30mph in 2nd gear it's only turning 2500rpm.  Either your car is geared REALLY tall or the speedo is reading faster than actual speed.  I'd compare it to a GPS speed to verify. 

2500 rpm

 wont hurt it

Dont adjust idle. that's retarded and will waste tons of gas. Obviously the solution is to keep it in a lower gear. Try to keep the RPM above 900 when coasting, and above 1200-1500 when accelerating gently. If you can coast at 1000 rpm that's probably fine, but you'd need to accelerate gently so you dont lug the engine. If you have to, just leave it in 2nd gear. The engine wont mind doing 2500 rpm for a while. It does that on the highway anyway probably.

Adjust idle so that it's close to 1200 RPMs and downshift and just idle through town.  I had the same problem with mine

you COULD change the rear-end gear ratio, but that isn't cheap.  OR get bigger rear tires, that would reduce the 2500 rpm in second some ....................................