Strangely only on the road, must be a suspension thing.
Ok, Sooooo ,,, My last post was a little attempt at humor but this one is going to be a little more on the serious side.
I knew that the 140 to 150 numbers you quoted were getting up there a bit but after doing a Google search to find out what MPH range you're really talking about, WOW dude! I'll have to say that in my honest opinion, I think you're asking a lot from a KLR. You're complaining about a vibration at 87 to 92 MPH (give or take a bit in translation). That is not a speed that any designer at Kawasaki ever intended the KLR to handle well at even with 100 percent road tires.
The things that make a KLR a good "all around" bike, are what make it a unsuitable bike at those speeds. Note: "UNSUITABLE" was not a typo. Sure you can do it on a KLR and I've done it a few times myself but it sure isn't pleasant and didn't last more than a mile or two. Back in my "lesser developed" days, I had a 1972 Ford Maverick with a 302, 4 barrel, with headers that I'd run down the road at somewhere around 130MPH, slowed it down after each run with my 4 wheel drum brakes and my mostly bald bias ply tires. I telling you about the Ford just to prove that dumb luck can get you by "for a while" and I, like most of us, I wasn't vaccinated against "stupid" as a child either.
My advice, IF THOSE ARE THE SPEEDS you wish to ride "safely" at, buy a second bike or trade off the KLR for something else. My Versys 1000 eats up those speeds without any difficulty and I'm sure that sooner or later I'll have a ticket or two to prove it. Like I said, dumb luck gets you by for a while. Both of my Goldwings of the past (1500 & 1800) would do that with the cruise control on, listening to classic rock. KLR's just get blown all over the road at high speeds. You'll need a fat "bitch on the bike" just to hold the bike down. Sooooo,,,, Ditch the fat chick, get yourself a real road bike. Throw some mud on it so it looks like a KLR if that's the look you're after. You'll be better off in the long run than trying to make a KLR, a 90 MPH street bike. Remember, you can always tow the KLR on a trailer behind a Wing but you have to stop for ice cream and FN Wingers like to hug, a lot, no, really, a lot so you'll get the Covid virus for sure. Food for thought.