vibrations third gear

dear oversea friends,
I drive a Jeep Grand Wagoneer with a TF 727 transmission. I did an overhaul on the transmission about 5.000 miles ago. It has worked fine all the time! I just returned from vacation in Spain, coming from the netherlands, which is about 2.000 miles both ways. The trans shifted fantastic, smooth, kick-down works perfect, downshifts nicely at steep hills on the highway. After the first day that we returned, all of a sudden the third gear started vibrating when coming from second gear. When de revs go further up, the vibrating dissapears. I tried the same only driving in first or second, but no vibrations?? Wghat could it possibly be? Any clues?