Which IMU, any at all?

Ground rover with long wireless range (1/4 mile or more depending on obstacles), will have GPS on it for absolute location.

Which IMU would work best and is easiest to process the output of?

9DOF Razor -> https://www.sparkfun.com/products/10736

ArduIMU V3 -> DIYDrones ArduIMU+ V3 - DEV-11055 - SparkFun Electronics

I'm looking for the following outputs at the minimum:

pitch
yaw/heading
roll

Basically, from 1/4 mile away, with no camera imagery, I want to be able to know the exact position of the rover- The GPS lat/long are a given, but I want to know if it's halfway onto it's side climbing a steep angled slope, what direction it's pointing, and things like that.

With that said, do I really need a full IMU, or would I be wasting my money, better off to get a 3 axis Gyro or accelerometer?

For 'absolute' orientation I think your best bet would be a 3-axis compass (magnetometer).

The MPU6050 is quite a cheap and nice 6 DOF IMU. If you want a Magnetometer to get 9 DOF, maybe try the HMC5883L:

http://bildr.org/2012/02/hmc5883l_arduino/

When ordering in China, you can get it all for around 20$.

I'm trying to figure out how all 3 components in 9DOF work together, based on this post: Magnetometers and Accelerometers and Gyros (oh my?) - SparkFun Electronics

accelerometer + trigonometery = where 'down' is

magnetometer = where magnetic north is (what do the other 2 axis do??)

gyro = measures how quickly it is moving pitch/yaw/roll (which I really don't really need to know, I don't think? I want it's position, not how fast it's getting there)

accelerometer + trigonometery = where 'down' is

Only works when there is no other acceleration. Can't tell the difference between standing still and accelerating toward earth at 2g while upside-down.

so how exactly does the IMU provide positional data? I'm trying to wrap my mind around this concept...

Ok so I think this is how it works.
With the accelerometer, you can find your acceleration and where down is with a gyro.
The gyro helps the accelerometer determine its orientation, and therefore figure out where down is.
Magnetometer is dependent on where you are located on the earth, because the magnet field is different, but it is suppose to assist the gyro with the rotation data (i think)

With some magical 3D vector trig, you can find your rotational and acceleration data (which includes where down is).

so what does an IMU put out, data wise?

I'm only going to poll the data sensors every 10 seconds or so, and simply putting the data onto a display screen on the controller. I just want to be able to know where it is incase I have a mechanical failure, or if it's about to go turtle from rough terrain. The rover's top speed is 3 MPH and I'll be downthrottled if It needs to crawl rough terrain. Powered by 1800W of electric motors and using six ATV tires, it won't be getting stuck unless it breaks the driveline. But that's where I might want to know if it's trying to go turtle, say, if it's climbing a slope that's 40 degrees inclined [pitch] and 30 degrees slanted [roll].

An IMU is usually a board that combines all three sensors and outputs either raw data directly from all three sensors or a filtered data that usually tells you heading, rotation, and acceleration.

For example, the high end IMUs (the links you posted) contain an onboard chip (ATMega8) that filters the data for you so you don't need to deal with the complicated math and etc.

As for your application, you can get by with only a gyroscope or accelerometer. However, for better and more accurate data, a combination of both will be more useful. If you want easy coding, just get an IMU (either one will work from your links) as it will give you the data you want directly (tilt and pitch).