[rotorwrench]

The one to one drive locks the input shaft to the output shaft. This connection could be where the problem is. The 2nd/high gear doesn't engage the input shaft all that much since it just dogs on there. Deceleration and especially down hill puts an engine brake load on the drive train. When this happens, the gears, splines, & drive line components start to ride on the opposite surfaces from where they do under acceleration. This is a different strain on things and it can change thrust loads inside the transmission and rear axle. If the input shaft can move too much, it may cause a decouple of the 1:1 drive. If the gear is sloppy on the shift fork or main output spline then that can have an affect.

You can generally feel if the shift lever detents are working. It takes a bit of a push or pull to overcome them. If they feel OK than that might not be the problem.