I would suspect the starter too... but can do quick test to verify if it is
the starter... with a remote start... you can hook up positive to battery
and the other wire to the purple wire by the passenger side engine bay where
you see strip of wires into a strip connector.
Another possibility is bad ignition switch... does it have a strong return
from trying to crank? Does it feels funny?
The last possibility is bad overload relay... but this should not prevent
you from cranking... it will however prevent you from starting on gas
engine... not sure on diesel.