Good luck, and hope it turns out to be something simple and easy to fix/resolve. However, it does 'sound' as though it's given up for some 'part' that is small or relatively inexpensive in itself, but is somewhere inside the gearbox which does mean it all has to come out and be overhauled. That of course costs substantial money.
Some gearboxes last for 200K plus miles and other's fail at low miles, there's just no way of knowing or telling if something will give. Fluid change, or no fluid change doesn't seem to be a factor either as some of those ultra high miled boxes have had no change and some of the low miler's have had the change.