1998 is old
Defender came out in 1990 and ended production in 2016. 18 years with TCS vs 8 without. Even if you include the Ninety/One Ten that only brings it up 14 years without TCS. So the majority of 90's & 110's have TCS more than likely.
No idea what you are talking about with regards to the centre diff. No need to upgrade at all. Are you getting confused with how it works and what it does??
As for pulling over and having both wheels on one side on the grass. I'd suggest more likely they didn't have it in difflock. Hugely unlikely that it couldn't have driven off, unless the wheels where in a ditch and it was sat on the radius arms, axle & chassis.
In lab conditions on rollers or ice maybe it would be possible or if on a super steep climb. But real life it would be hugely unlikely to not have ability to drive off. You could also opt to go backwards as well as forwards and turn the wheels left to right. So I either don't believe you or there is more going on with that story.