You'd probably help yourself to spend some time on Google and YouTube and learn how a differential works. Then specifically the D2.
The D2 has open diffs in the axles and without a centre difflock you have an open in the centre. A normal 4wd system such as in a Series Land Rover or older Jeeps, when you put it in 4wd it locks the front and rear propshafts together so they rotate at the same speed. The centre diff lock does essentially the same thing by locking the front & rear props together.
Without the centre diff lock the front and rear props can rotate at different speeds. But the diff will send power to the axle with least resistance while splitting torque equally. So if one prop is taking all the power the TCS will brake the wheels on that axle to try and force the other prop to rotate.
This works with varying degress of success. Mostly the TCS works way better with the centre diff lock, hence it became available on some D2 models and every model since has either a lockable or limited slip centre diff. Some conditions will make it very hard for the TCS to overcome the load and rotate both axles or all 4 wheels when all 3 diffs are open.
However, should the TCS not be working, then you are in a 1wd vehicle and it will be mostly rubbish off road.
All the ones I saw had CDL so did not realise it was optional, was looking at as new as possible.