First you should know that a "locking diff" is not the same as "limited slip diff" (lsd)--sometimes the terms are incorrectly invoked as interchangeable. I do not think the 240 ever had a locking rear--but an "lsd" is a possibility as it was an option through the years. With the lsd rear there is a clutch system which ties the two axles together. If the car is raised in the rear and you spin one tire by hand with a normal open differential the other tire will spin in the opposite direction. With an lsd rear the two tires will spin in unison.
Some 940's had a "locking rear" which would mechanically lock the axles together under certain conditions. The tire spinning check does not work with the locking rear--they will behave as a normal open rear when the tire is hand spun. I found this out when I had a '92 940 Turbo which had been rear ended while parked. When I first got the car I did the hand spin check (not knowing about the locking type) to see if it had lsd. After the impact I decided to spin the l/r wheel up to check that it hadn't been damaged. With the left rear jacked up I started the car, put it in drive and then stepped out to look at the wheel spinning. Much to my surprise the car started doing a little hop with the r/r wheel also turning (slipping on dirt) and the front wheels chocked. That's when I did some research and found the existence of the "locker" which is set to come into play in the 5-30mph range.
The purpose of either system is to get power going to both rear wheels in slippery conditions--especially useful if only one tire is on a slick surface. I've read of modifications that can be made to the "locker" which enables it beyond the 30mph range for performance purposes--ok for drag racing I guess --but on a road course I wouldn't want that system locking and unlocking in the middle of a turn. I've added a used lsd unit to my sometime track car '69 144S and it made a world of difference in certain turns where the inside wheel becomes unloaded and will spin even in the higher gears. -- Dave