I've had the same problem. Mine came at 55 then went away until 70 then it came back until 75, then it was gone again. I put new tires on and for the most part it's gone away. I'm thinking there could be a couple of causes for this. One the steering stabilizer (in conjunction with out of balance tires). My idea goes like this; You've got out of balance tires that start to wobble at high speeds. This causes the stabilizer to become worn out, but not totally shot. If you just put another set of tires on, without being balanced, they are going to be slightly out of balance, causing the problem to continue. So just putting tires on from another vehicle won't fix the problem. And you think it can't be the tires. But it probably still is, along with the stabilizer.
Another thing to consider (and this also goes with the stabilizer being bad) Most of our larger, even stock, tire sizes don't meet UTQGS (Uniform Tire Quality Grade Standards). Basically, truck tires don't meet the same standards as car tires. So even if you've had your tires "high speed" balanced, this may go over the 60mph speed where the wobble exists.
So for me, the next thing I do is replace the stabilizer.
I need to correct my error. i.e. Lack of knowledge. Just got my new Tranny mount in the mail, it's not solid like it looked in the picture or like a motor mount. So back to the question of how do you know your tranny mount has gone south. IDK ?
Wanted to update everyone on the Tranny mount. I could move the transfer case up and down a little, but it was really hard to tell it was bad...until i swapped them. I couldn't get a socket on the nut or bolt on the old one, so i had to take the crossmember off. When tightening the new one, with crossmember back in place and weight on mount, i could easily get to the bolt of the tranny mount. So i'd say this would be a good indication if your mount is worn out. In hindsight, i could've done this replacement without taking crossmember out by jacking up the Tranny some more, but wasn't sure how high I could go at the time. Took it for a test spin around town and, maybe it's just in my head, but it felt alot better. I didn't get up to 65-70 yet to see if it helped with vibes yet but will test that today or tomorrow.
Vibe is still there, but i've also got a "clunk" noise back there when i go over bumps. Can't feel anything lose, but prolly control
arm bushings. Don't want to buy bushing kit yet because i'm this close ( picture thumb and forefinger 1/4 " apart ) from getting the RC 3.5" lift and it comes with LCA's.