I usually see bikes freestanding (with ratchet straps or something similar) in the backs of pickups. I have also seen a simple rack made from 2"x4"s in the bed behind the cab. There's bound to be a simple DIY answer for you somewhere. Try these for inspiration:
How to Build a Bike Rack for a Truck
How to Build a Bike Rack for a Pickup Truck
How To Build a PVC Bike Rack
How to Build Your Own Bike Carrier
Good luck, keep us posted.