They have shut down the km 21 and all the other "garitas aduanales" to much corruption and hassling the Mexicans for mordidas and made up charges of smuggling, the gringos were normally let through with out too much hassle, we used to slow down, they saw the US plate and waived us through.
There used to be retenes aduanales in just about every state in the mid 60's to the early 70's, my dad used to ask them what country we were entering.
The only one left is the Sonoyta check point, they are the worst of the worst in corruption schemes. 
navegator