Re: 2001 GC 4.0 dies at high speeds.
If it dies just suddenly, i would not expect the fuel supply to be the problem.
The 4.0 has a cable loom which runs around the back side of the cylinder head, where it can shave on a bolt head and cause sudden short. I would look for that first.
If nothing found and no other fault codes set, i would swap the auto shutdown relay with the transmission control relay in the PDC (power distribution center)under the hood.
The problem can be sensor related, such as crank or cam sensor, but without codes it will become a hit and miss game.