theres a way on most trucks to have the tailgate held up partially at an angle, ive done this with all my trucks in the past, you still need to make sure bike is secure and only really works if your rear wheel isn't fully on the tailgate. Might give you enough clearance.
You just have to route the cables around the latch. see pic below:

You just have to route the cables around the latch. see pic below:
