one word of warning – I’d be surprised if a 20-year old LSD really had much ‘lock’ left in it. Most LSDs work around a constantly-engaged clutch, which naturally wears over time (it is being slipped every time you’re not driving straight ahead on a completely flat road). If you’ve got it off-vehicle then it’d be worth replacing the clutch pack within it before you fit it to a new van.
incidentally, you can get a lot of the effect of an LSD (in the ‘getting out of a muddy field’ sense) in a RWD vehicle by applying the handbrake loosely. If you’ve got one wheel with zero grip but the other with some grip, applying the handbrake gently allows the car to push some of the torque down the driven wheel, as the non-driven wheel is being resisted by the brake. Modern e-diffs work in a similar way (but, smartly, not braking the wheel with grip at the same time as the wheel without grip), but I’ve used this method with some success whilst driving across sandy bits of the sahara..