Is it true if you use eco mode it will eventually damage your engine?
Heh there Mr. Blue, good to have you here! When you get a chance, post up on the
New Member section

:
https://www.durangosrt.org/forums/new-member-introduction.2/
To answer your question, the short answer is
NO.
The 4 Cyl Mode (aka "MDS" or "Multi-Displacement-System") will not damage your engine:
It might be a nuisance if you don't like the "feel" of it.
If that's the case, go to
DRIVE MODES and set up the
CUSTOM MODE to anything but "
ECO".
After that, just tap the dash mounted SRT button twice and it will go directly into
CUSTOM MODE.
The
4 CYLINDER/ECO/MDS MODE will go away.

(you have to do this every time you start the vehicle)