Before you try another pin box, have you tried adjusting the height of your hitch? We had to move our hitch up one notch when we bought our 07 GMC. The new bed style in 07 has higher rails than the old style. It might not work for you, but it could be worth a try. You just have to be careful not to raise the hitch so high that it messes up the trailer angle for towing.
We replaced our pin box with a Demco which works on a different principle than the air or shock pin boxes like Trail Air and 5th Airborn. It does a great job for us in handling the back and forth shocks you get from starting stopping and uneven road. The Trail Air and 5th Airborn work on the up and down motion. If you do a search for
Demco and you can see a video of how it works.
Good Luck,
Kristy
PS I see you spend time in San Luis Obispo. I love it there and wish it weren't so far from us! It is beautiful and so close to some great places to visit.