2000 v70xc the car won't move in reverse car has 250k on it and the fluid has been changed. What could it be.

 

did it work before the fluid change?

start by checking the fluid level (using the correct procedure - there's vids on youtube). what was done for the fluid change ie drain/fill, flush by removing the cooler lines etc? Do you know what kind of fluid was used during the change (see your owners manual for the proper fluid specs for the AWD (I believe its JWS 3309)