Matthew, welcome to the forums. First, to answer your question it should be easy to fit the roverdrive - see their faq
However, it would be best to establish what you actually have first. I feel like people are (unfairly) having a little fun with you here, so let me start by saying your truck is not the year you think it is. Pictures speak volumes so a few pictures of things like the shifter knob and interior, engine compartment, perhaps one of the transmission/transfer case can help others tell you what you actually have so you don't get bad advice.
The point Red90 was making is that on land rovers there is a transmission which works as you normally know it, and then a transfer case attached to it which allows you to do two things:
1. Engage a differential to send power to both the front and rear
2. Change to low range, effectively another (lower) set of gear ratios in all gears
The roverdrive connects to the back of the transfer case and does the opposite - gives you a high range, effectively another (higher) set of gear ratios in all gears.
In case it wasn't clear, he was saying that all defender transfer cases have the same low range gear ratios unless there is some custom gearing in it so it seems odd that it is billed as a custom offroad transfer case (and likely isn't). The differences between them are in the high range which is what you are experiencing. The gear ratio is expressed as a ratio, say 1:1.667, 1:1.2, 1:1.4 etc and this means that for every revolution on the engine, the driveshaft will turn the corresponding amount. As the number on the right gets bigger, the lower your rpms will be at highway speeds. There are online gear ratio calculators online that can help you calculate your rpms at speed, or figure out your gear ratio at a given speed knowing your tire size and differential ratios in your axles. You should be able to determine the gear ratios from the serial numbers on the transfer case as well which is why people were asking.
The diff ratios in the axles are important and likely a factor in your situation. These can be determined by jacking up the axles until the wheels are off the ground, putting the transfer case in neutral, and counting the wheel revolutions for the driveshaft revolutions (same principle as the transfer ratios described above).
Short term the roverdrive may be an easy fix, but long term you are better off figuring out your current ratios and where they are (axles or transfer case) and correcting them there (swapping diffs or the transfer case itself).