It could have been doing it all along and I did not notice.

Since my decatification and new exhaust my truck pops through exhaust.

It only happens with torque convertor locked, coasting, I apply the brake and the torque convertor unlocks, the tach goes from about 1000 rpm to about 500 or so then back to about 800 and truck pops. I can hear it in the exhaust almost like a backfire on a gasser, but not so loud. No codes stored, no SES light. Truck nomally idles at about 575RPM when warm, good smooth idle.
Any ideas?

What, if any, muffler do you have? Glass packed or straight pipe will give a noise like that.

Jardine muffler I believe.. straight through...this is normal then?

It sounds normal to me, my truck had dual glass packs when I bought it and it did that, and my old '79 diesel rabbit made A LOT of popping noise with a Magnaflow muffler that was straight through.