The Lexus GX460, Land Cruiser Prado, and 4Runner were all built on the same platform and all built in the same factory in Japan.
The Lexus LX600, Toyota Land Cruiser 300 series, 2024 Lexus GX, 2024 NA Land Cruiser, Prado, Tundra, and Sequoia are all built on slightly dimensionally different TNGA-F platform which is stretched to accommodate differences in the specific vehicle requirements e.g. the much longer wheelbase required for a Tundra.
The current speculation is that with the new 2024 Tacoma as well as Grand Highlander and Lexus TX (unibody crossover) going into production in the US, the 4Runner production will remain in Japan. 4Runner is also largely assumed to be the lowest of priority in terms of updating and/or release date.
Dimensionally there is only about 2 or 3 inches separating the GX from the full fat 300 series based LX in width and length which has led some to question whether the LX600 will remain viable in the US market or go the way of the 200 series Land Cruiser. So, I think you would be fine space wise in the new GX. It is longer than a Grenadier and while I don't have the specs off hand, should be similar in terms of cargo volume. Certainly more than the 4 door JK you used to have.
Regarding 4Runner vs future NA Land Cruiser having the same underpinnings, the 4Runner, if it ever gets updated, is speculated to have the 4 cylinder turbo and rest of the drivetrain of the newly released Tacoma and not the twin turbo V6 torque monster of the GX. The NA Land Cruiser is expected to slot above the 4Runner and may have the V6 available.
I certainly eagerly await the specs when they become available. I don't really eagerly await the pricing though.