When I cross my quad fom TJ to SD they just ask for the registration, the same from SD to TJ.
When I crossed the Bronco I bought in TJ to register in SD they sent me to secondary to inspect the vehicle and asked for the Pink Slip or registration and the Bill of Sale. I also bought my GF's car in TJ and the agent in the booth asked for the Pink Slip and gave me a hard time because I only had the bill of sale. He checked the VIN# to make sure the car wasn't stolen before letting me cross into the states and advised that next time I have the Pink Slip in hand.
The majority of instances a DMV Bill of Sale (the Yellow ones NOT the ones you can print from the internet from their website) with your info and the sellers (could be the shop your truck was in) is enough. If you are going to cross the border from San Ysidro, make sure you cross in the last 2 lanes on your left hand side (where the curios and big screens are next to Mex Customs/Aduana) because those are the widest lanes where a trailer can go by and after you cross the booth the exit lanes are wide. If you take other lanes it is harder to manuever a trailer through.
If you cross through Otay it will take longer, the agents are hardasses and take forever to inspect your ride and document your entrance/exit.
Hope this helps and good luck!!