You can compress the shock by hand, if it's even a gas charged shock at all. Sometimes you have to lean on them a little, but they'll move.
No need to jack anything up. In fact on a number of vehicles the shock retains the spring unless there's weight on the suspension. I don't know what your 4x4 conversion looks like, but generally, don't jack up a coil spring conversion and pull the shock.
'99 EB ex ENG KSWB news van, low rent 4x4 conversion (mostly fixed by now), home built interior.