Ive seen this caused by two different things on stock trucks first one was an injector problem not to many dealers willing to pony up $6000.00 dollar warranty injectors without beeing able to pull a code which it didnt on this vehicle.. And the other was a fuel pressure sensor module and it didnt set a code either basicly the sensor wasn't reading normal and had the fuel pressure holding high causing high idle or throttle sticking..

Regardless of what they tell you non of this SH#T is normal. But allot of dealers dont want to replace something if they cant get a warranty failure code.. Do this if they drive and tell you its normal again. Ask the service manager to ride with you to compare it to another one the lot..

Now just a side note Diesels dont come down from high rpm like a gas engine does. I don't know how knew you are to diesels but just wanted to make sure you know. If your sitting in your driveway and rev it to 3000 rpm and let off it returns to idle very slowly

Hope this helps
Iz