The vibration between 67 and 70 mph sounds like an unbalanced drive shaft. If it goes away after 70 it probably is. Sometimes unbalanced spinning objects noticebly vibrate at certain multiples. If you were able to go fast enough the vibration would probably return, go faster yet it would go away, then even faster it would come back again. But you are probably only feeling the first multiple and might not get to the next one it shakes at because it is too fast. Or this could be the first multiple that is felt, there may be others preceding it but the speed is slower so they are not felt. Little trivial tidbit that I learned from a journeyman about balancing industrial shafts, rotors and turbines. Can't help you with the second gear question. Is it only in second gear that it vibrates or is it at a certain speed that just so happens to be high RPMs in second? If it has to do with speed and not nesasarally (that is spelled wrong) 2nd gear it could be one of the driveshaft's multiples of inbalance. Could be that you only feel the vibration because in 2nd and high RPMs there is a lot more force to make the vibration felt even more. But that is my wild stab in the dark.
OP, like he said^^ I doubt its your clutch(err...hope its not at least)...But my guess would be that your wheels are imbalanced. I used to work at discount tire for a couple years and I would hear this all the time, "but, but , but sir...it only vibrates at this certain speed, etc."(not making fun of you, just all the people who would come in w/ this prob)
I would take it to discount or your favorite tire shop, and have them zero out all of the tires. Make sure you go into the garage or can at least see the guys balance your tires off, b/c some times...err a lot of times they will get "close" and just throw them on.. So imo, I would do that first, b/c it is cheap and could probably use it anyways; and than go from there. But I really do think that it is something besides your clutch, either way, good luck:bigthumbsup
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