SCS,
There are several possible causes, and sometimes it's difficult to run this down. An exhaust leak near the manifold end of the system will sometimes contribute, as will a carburetter which is oversupplying fuel due to a stuck float valve or a misadjusted float. But, the two most common problems are a bad diverter valve, which shows as N/A in the catalog, and a bad exhaust check valve, which is shown as still supplied. There are two versions, with the aftermarket one being considerably less expensive.
As stated, there are many things which can make an engine backfire on decelleration. These are simply the most common. I hope this helps. By the way, where are you in Colorado? My shop is in Fort Collins.
Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon