My 1996 Range Rover won't shift through all gears

Have the trouble codes pulled to see if it si electrical problem. Any Advance Auto or Auto Zone will do this for free.
If no codes come up, take it to a transmission shop as you may need to rebuild the transmission. The torque converter or a clutch pack has probably failed if no trouble codes come up. If a code does come up it will likely be a solenoid which a transmission shop can repair as well. Take the vehicle to a transmission shop and not a standard repair shop. AAMCO is a bit overpriced. Automatic transmissions require special tools and other tools. Taking to a regular mechanic, they will take it out and send it to transmission shop so you end up paying more for repair.