First do a test on the ficm voltage. If that is fine then do a power balance test on the injectors to see how they are on the chart. This is the best and easiest way to test the injectors. Is there any codes at all. I've had some in the shop with the unison ring seizing up in the turbos or even a cat that is plugging up causing the hesitation. A good diesel shop should tell you in 1 hour on what it is.
Just my two cents

unfortunately, sometimes when you take a vehilce to a shop you just have to :moon: and take it....sorry for you troubles man.

I'm a diesel tech myself and believe me there is nothing I hate more than an ignorant parts changer that can't actually diagnose and work out the issues...... however sometimes on these 6.0s you actually do have to spend some big coin fixing the smaller issue to get a definitive answer on what is actually going on ya know what I'm sayin? I obviously wasn't there and not disagreeing with you just thought maybe having a little more insight would ,make you feel better instead of feeling frawed if you actually werent thats all