Ah, so your 8.4 was your blood glucose, as opposed to your urine?
Working on the assumption it's blood, it would very much depend when you tested and what you had eaten. By that's I mean, if your test was, say, 45 minutes after you ate something, then it could be fine. If it were a couple of hours after you'd eaten it might be considered a bit high. The link following explains blood sugar levels a bit more.
http://www.diabetes.co.uk/diabetes_care/blood-sugar-level-ranges.html Whilst the post meal level for T2s is 8.5, many try to be a bit lower at the 2 hour point.
In terms of why you might be feeling tired. That's a tricky one as so many things can impact on our levels of fatigue, but within the diabetic communities, high levels can do that, but then so can being generally tired, or thyroid issues, and several other things.
It's certainly good that you are doing some testing, but the results need to be viewed with a bit of context.
Finally Diabetes Insipidus isn't very common, as I'm sure you already know. To avoid to many queries, such as my own, it might be useful if your profile included the medication you take. It's up to you.
Good luck with it all.