That too has probably allowed or helped this XC to go into whatever it is doing
Look, it's never been serviced. To get over 300K miles out of it is a friggen miracle!

Just shop for a used transmission or if you have a QUALIFIED shop near you, rebuild yours, but I caution that the case itself may be too (internally) worn out to even do that!.

(I've come across several of these, with less miles than yours, that could not be rebuilt)!