These are the main ones I hear around ny and nj lately. Encryption is in use about 99.9% of the time. I believe the Postal Police still use the fed trunked system in NYC alot.
While I have no experience with this particular agency, many federal agencies have a portable or nationwide frequency pair exclusively for tactical deployment use and not for everyday use. So don't limit yourself to known frequencies, but also search the immediate spectrum like ecps92 mentioned.
I have been searching the 406-420 band sents this thread started and have not hit on anything.
I am going to Opelika friday and will stop at the postoffice to see if there still there.
I made it to Opelika today and stopped by the post office and the mobile command center
vehicle is still there in the parking lot in back. There was a mailman loading mail in a truck so I
stop and ask him about the vehicle. He saw my 396 in my shirt pocket and he turn out to be a
scanner nut too. He said no one had been in it all week but they was still useing. He also said
talk was it and the inspectors was not going any were soon. Back to searching the 406-420
band I go.