John -- Fixite has the right idea. I think you are tailgating the hard way
A PTO-driven hoist should work just fine for you. You are using tail gate chains, right??
The trick is setting your gate chains
When I spread that way (bank run, gravel, recycled asphalt millings, 2"" recycled concrete, etc), I set my tail gate chains for the amount of gate opening I want, then get the truck about 6 feet from where I want the spread to start, raise the bed until I feel the load shift aft and load the tailgate ... Then I ease off in 1st gear while simultaneously releasing the the tailgate lock. Um, yes -- you will want to be sure your chains are good and the chain locks are solid -- it's a mighty "whunk!!" when that gate comes open!
Maintain a steady pace. As you go, the bed will continue to raise until full up. At that point, set the lift control to "hold" so the hydraulic pump doesn't over pressure. Keep rolling until the bed is empty. Take the PTO out of gear, lower the bed, lock the gate, go reload.
It might take you a few tries to learn how much chain to give the gate.
Of course this is easier if you have an air locked gate ...
but it might give you a better idea even with a manual gate lock.
I get consistent results like this. This is a job I did recently at work (357 Peterbilt dumptruck -- 1st pass
):
Tailgate chains? 
It'll be way easier to build an adjustable tailgate than to re-power the hoist. Plus it should actually work.