My 1998 Toyota LandCruiser is stalling when shifting into gear

Torque converters have been known to seize and, if that’s the case, it could stall the engine because there’s no slip to allow the engine to idle while the car is stationary. You need to have this checked by a transmission specialist who may have seen this very thing before. Either way, the vehicle won’t be going anywhere under its own power.

Even if the gearbox itself had frozen solid, the torque converter should still allow enough slip to let the engine idle. So that’s definitely where to start the investigation.