I've done this with a 25 year old Land Rover. I did a lot of research when I bought my Rover, and studied the process.
You have to understand a few things up front. First, there is the difference between Federal Law and State Law. Federal controls the importation into the country. State controls the operation of the vehicle, licensing, things like that. This is why there are "safety rules" regarding vehicle type importation, and the operation of ATVs and such on highways. It also means, for instance, yes, you can import that 25 year old diesel Land Rover, but you are NOT registering it for use in California.
Vehicles over 25 years old are considered "safe" in that they are not likely to be used enough (or in enough volume) to be a widespread hazard on the road.
"Newer" vehicles are considered a "volume/hazard" since the '80s. If you remember the 80s, the Deutschmark (and several other currencies) were at low value compared to the dollar. This led to the import of many desirable "gray market" cars like Mercedes and Porsches which could be bought in Germany (as an example) for far less real cost than purchasing the car in the US. This caused many problems, so federal law on importation was changed.
So as it stands today, for a car less than 25 years old, it must be listed on the NHTSA "good list" as meeting the same standards as the same car sold in the US. If it's on the list, it then must be imported by a registered importer, who will make the appropriate modifications to meet US road standards. These modifications usually concern things like tail light color, bumpers, things like that. This list can be found in Appendix A to Part 593—List of Vehicles Determined to be Eligible for Importation (link:
https://www.ecfr.gov/current/title-49/subtitle-B/chapter-V/part-593/appendix-Appendix A to Part 593).
For the Land Cruiser, I see:
Toyota Land Cruiser (manufactured prior to9/1/2006) IFS 100 series 1999-2006
So you can beat the 25 year rule there.
You can petition NHTSA for an exemption, but you'll find things like air bags, back up cameras, things like that may not be required in some foreign markets, but are required in the US, so they would fail in that case. I don't remember the year that back up cameras became a US requirement, but if they are not on the "newer" import you want, it's not coming unless they are added. One of the issues with Land Rovers, and the reason they stopped offering a US spec Defender in 1998 is 1998 was the year air bags were required, and there was no way Rover was going to be able to meet that change. I'm not smart enough on Land Cruisers to know what they have and what they need, but 2006 seems to be significant.
Oh, and one other thing on the 25 year rule: It's 25 years from the date of manufacture, NOT 25 years from the model year. Proving date of manufacture "to the day" can be a bit sketchy... but "date first registered" is a good substitute.
That's the best I have for a précis, but if there are other questions, I'll take a swing at it.