Its normanl. My has always done it. Try to let off the throttle gradually and it will not spike so much. Think about what is happening . When you are on the throttle fuel is flowing at a required rate. Once you close the throttle the fuel is still in the line and flowing for a few seconds before the fuel is shut off or decreased to a lessor flow. That is where the spike comes from. That's the way it was explained to me. Hope this helps.
2010 - 2500 SLT; CCSB; 4x4; 6.7L; 3.73 LSD; H&S - Mini Maxx w/OD; Rail Plug; MCC Tuning by Dave Matthews; SUNCOAST- Billet Mega 68; II - SB66(66/74/14), 100hp Nozzles; FLOOR IT - CP3 Kit; ATS - 3rd Gen. Manifold; SHIBBY - Stage II Horn, GHD w/Re-locator, Boost Tube; ARP - 425s; FLOPRO - 4" Turbo Back w/Hushpower muffler; FASS - Titanium 150; S&B; DAYSTAR - 2" Level; BILSTEIN - 5100s; PSC - SBS; K&N CCV Breather; PIAAs; ISSPRO - EV2 4th Gen; TOYO - AT IIs 35x12.50x17.
GONE....but not forgotton