On a similar vein to the expensive solution, it may be possible to make something with a ramp and hi-lift jack. Load on the tail gate wouldn't be much different than loading with a ramp (maybe a hundred pounds more?). Rough idea is a removable chock at top of ramp. Use bike power to drive bike into chock. Use hi-lift jack to lift tail of ramp above truck bed so now bike can roll downhill into truck. There are a few kinks to be worked out. Biggest one is how to get the chock out of the way (probably install a rear wheel support, remove chock and then roll into truck?). You will also need to change ramp/truck connection a little and add a larger vertical face as it isn't designed to be secure in that orientation.