Toyota never certified the 70 series vehicles with the USA EPA and NTHSA. If you try to import a LandCruiser 78 Troop Carrier, you have to bring it into compliance with the applicable emissions and safety equipment within a set period of time (6 months? a year?). If you are not able to get the needed approvals, then the vehicle has to be exported or destroyed.
There are importers who specialize in bringing vehicles up to USA code. The process is very expensive. For a vehicle from the 70 series, this could mean crash testing to verify that the vehicle's bumper and frame absorb crash energy the way the government has mandated. This might make sense if you plan to import a large number of vehicles, but for a single truck it's way too expensive.
There is provision in the laws for importing a vehicle over a certain age. For EPA, vehicles over 21 years of age don't need to comply with emissions rules. For NHTSA, the vehicle age is 25 years. Since you have to meet both sets of regulations, the 25 year period is the rule you hear mentioned.
So if you can find a Land Cruiser 78 that was manufactured in 1984 or early 1985, you can import it without much additional expense.
Once a vehicle clears customs, it still has to pass inspection in the state in which you plan to register it. Some states are very lax about inspections. Others can be very fussy. California has very strict inspections for older vehicles, for example. So you might have to do additional work (or file petitions) to get the OK from your state DMV.
Occasionally you'll find a 70 series vehicle with a USA license plate. Proffitt's Cruisers in Colorado owns one. You might contact them to find out how they managed to do it.
Frankly, you'd spend a lot less money by finding a LandCruiser 45 or 55 in ok condition. These vehicles were imported to USA by Toyota, so they are legal. Then pay a company like Proffitt's to put fix it up, maybe including the installation of a diesel engine.
Here's an FJ55 in Atlanta for $1500
http://atlanta.craigslist.org/nat/cto/2072527445.html
Here's one in LA for $1700
http://losangeles.craigslist.org/sfv/cto/2066549811.html
Here's one without engine in Spokane for $650
http://spokane.craigslist.org/cto/2071673245.html
