Use come-alongs or strapping to secure it to the pallet, and cover the whole thing with a tarp or sheet plastic. I see a TON of engines and trans on LTL trucks, and this is usually how they are loaded.
Make sure you read over all the policies and fees that the shipping company charges...some of them charge a fee if you can't unload the pallet yourself, and lifting a 300-400 lbs engine down from a semi trailer is NOT a one person job...