Your problem:
You are managed against a metric (x # of pallets to unload in y amount of time) that is invalid due to the way the trucks are packed, and the shipments are scheduled.
The store's problems:
(a) The shipping party is not working with the recieving party to ensure that the right resources are available to perform the tasks of unloading all the pallets when the trucks come in. The proof of this is you being overloaded and the other crew having a circle jerk around an empty truck.
(b) The performance metrics you are being measured against are not valid as the truck shipments vary.
The impact to you:
You look like a ****head and get fired.
Your plan to fix the problem:
Inform the company they are not operating to their potential efficency, and as a result are losing profits by spending too much money on staffing, as well as not moving the product to the floor quickly enough.
Who you need to contact to remedy the problem:
Contact HR regarding your performance evaluation against an invalid metric, and contact your store/regional operations manager regarding the other problem. You could try to work with your direct manager, but the problem is that you are going to make him look like a jackas here. Suggest to the ops. manager how they can fix the problem by adjusting augmenting your shift with more staff, which actually increases revenue by putting more product on the floor faster.
Other advice:
Document everything. You will need it if you get fired and seek unemployment or other remedies.
Be a model employee...be courteous, polite, and professional. Arrive to work early/on time, and leave late/at end of shift. Come to work clean and dressed appropriately.
Do not: mouth off, talk **** with other people about your boss, the company, etc., make enemies, take any bait that mgmt. lays out (e.g. encourage you to snap/walk off job/curse at your boss, etc.)
If that all works out you will be considered a 'visionary', 'team worker', and a 'problem-solver' instead of an underperformer...
And get yourself a copy of the "help wanted" while you are at it...! Good luck.