Bought 07 B3000 new couple months ago with 100k. Has vibration at 70KPH in overdrive only. Tires balanced rotated twice. Drive shaft swapped out. Tranny swapped out at cost to Mazda CA of 5000+. Ford & Mazda techs have both looked at it. They are now telling me that all the Rangers & B's have this vibe. Anyone else have this

 

I have had this for years, too, in my 2001 3L Trib. I haven't taken it in, though.
Interesting that so many, from your post, have it too.

 

My 07 has a thing. I've got a stupid tire thing going on, but it's just the stock wranglers. Kinda wallopy. The problem goes away when I put the winter tires on.......

 

I had this same problem with my 98 B3000. The drive shaft started vibrating at 75 mph. It caused the transmission oil seal to back out. The loss of fluid caused the tranny to burn out. The replacement transmission didn't vibrate until it reached 90 mph.
Maybe the problem is in the connection between the drive shaft and he tranny. Those splines are steal on steal and a minute amount of play would vibrate at high speeds.