Hey guys, need some help! I have a 2005 ford excursion with the 6.0 diesel. I've had it for a few years now. And recently has been giving me troubles! Sometimes when I start it up and try to put it in gear, nothing happens. I throw it in park again rev it some, then try again and sometimes it works and sometimes it doesn't. Can this be a vacuum issue or a solenoid or something?

Or do you think it's more serious than that? I also want to just add that when I got the truck a few years ago, I threw on a lift and bigger tires.