Maybe you shed a wheel weight?
Get the wheels ballanced and the tracking checked.
My sister has the same problem in a Ford Ka2, Its the tappets that become loose and frequently need to be tightened!! When they are so loose they make the steering wheel shake, or vibrate as you called it. My dad manages to tighten them each time, with no problem.. But it is slightly annoying!
I had the same problem in my old car (same speed too!) and I just needed to get my wheels balanced which cost about £10.
Hopefully (crosses fingers) should be solved tomorrow. The service/mot place didnt do it, but next on the list is replace a couple of worn tyres so i'll get the tyre place to balance it tomorrow.
Apparently if the tracking was off the wheels would be unevenly worn, and this isnt the case on mine so guessing it must be the balance.
I get that. At that speed. But I can trace that to engine vibe, and 70 being warp factor 9 in a defender.
Ah thats not too bad. Whenever i've been in a defender on the motorway its been towing a heavy trailer so havent seen it get that fast.
Really want to get one eventually.
I've done a fair few miles as a passenger in a friend's. Plus driven it off road quite a bit, and virtually lived out of it for a weekend. He's looking at replacing it with a newer one... just wish i had the money to buy this one off him and run it (it needs quite a lot of work). Its a 110 with 2 rows of seats plus rear benches, but gets used for carrying stuff around more than passengers. Think it would do up quite nicely tho