The Skylift is obtainable depending on how you prepare for the The Big One.
Won't go into too much detail, but need Skylift to take Train carriage and train itself.
Basically you see it at Sandy Shores as Michael when landed. Switch to Trevor and make your way to Sandy Shores Airfield. Keep switching between the 2 to make sure it doesn't despawn.
Save it as Trevor since it's his airfield.
UNFORTUNATELY, it is 50/50 of actually doing it. It explodes before arrival for me. However, I seen a couple of YouTube users actually achieve this.