Millerlightning

when coming down this long hill I would always shift the truck into neutral and just coast. I don't know why I but I I thought I was saving wear and tear on the engine or because the tach would go down to idle. 500ish. Now however I do the same thing the tachometer stays at the same level as if it were in gear. 2500ish. The truck is definitely in neutral because I can rev it past the level it's at and the tach goes up but comes back to the same cruising level, not the idle level. I just don't know what that means and why it has changed and is this a precursor to a costly trans repair? Truck is well maintained and just had the trans serviced/flushed about 15k miles ago and never went beyond mileage on any type of maintenance. This issue just started happening last week. Any insight and info will be much appreciated. Thanks