I got an engine light code of po401 and cylinders 3 and 7 misfiring, so I changed the EGR Valve, then the DTFE sensor and then the EGR Regulator solenoid. I changed the coil packs (all 8) and am still getting the same code for the EGR and still have the misfiring cylinders (3 and 7). Please help...

 

You did everything but look closely at the description of the 401 code.
It tells you low flow failure on the system test by the computer.
Think that suggests a blockage at some point?
To test for this, run engine at idle and apply some vacuum to the EGR valve.
If the passage is clear the engine idle will get rough and maybe stall telling you the passage is open or not. If not, that is your problem and ports behind the EGR need to be opened.
Once you clear the codes out, it takes two drive cycles to test the EGR again for a fault.
Treat the misfires as separate troubles until proven other wise.
BTW what year is this truck?
Good luck.