'01 Disco Vibration

I just bought a '01 Disco two weeks ago and I love it!! Noticed that when I bought the car all of the tires were new except the right rear. The other three tires still had the little rubber needles still on them that are on new tires. I also noticed that the right rear was cupped or flat spotted so I made a mental note to replace the tire. Over the holidays I drove it 840 miles round trip. On the way down on my trip I noticed that the right rear had a bad vibration in it on the interstate at 90mph. (don't worry I don't drive everywhere at 90mph!) When I got to my destination the next day I replaced the tire and some of the vibration went away but not all of it. This vibration only happened at about 90mph. I know disco's aren't designed to go fast and I maybe I am just learning the sounds and noises my new vehicle makes. Should I have the dealer look at the car or I am I just paranoid? It only has 44k miles on it and runs perfectly at all other conditions.

First, did you get the wheel balanced when you got the new tyre?

Second, as a quick check, raise the rear wheel off the ground and see if you can wobble the wheel. (worth checking both rear wheels) As it might be the wheel bearing starting to show wear. i.e. given time, the vibration and noise will increase to cover lower speeds.

Also, are all the tyres of the same brand/tyre patten, and are they road bias tryes, all terrain's or mud tyres?

Yes, It was balanced and the tire did match the other three. I will check out how tight the tire is. Thx alot!!