The problem is that the person who recommended to you the HD tembren was uninformed...Based on the same uninformed (as to TC's) recommendation, this same thing happened to a buddy with new dually Dodge and heavy Lance 1181 (heavier than your EC)...Same complaint as yours, but he then exchanged the HD's (no charge) for the standard tembren and the difference is huge, so now he's very satisfied...The HD's are rated at some 10k lbs and intended primarily for tow truck service, but in this case bigger is NOT better.
I've run the standard spec tembrens on my '07 dually Dodge and '09 EC 995 for nearly 5 yrs now, and 4yrs on previous rig and have been satisfied...However, recently I added two rear leafs (per side) due to extremely rough concrete freeway I-5 north out of Bakersfield (even caused passenger rear fastgun to pop loose!)...With the new leafs ($$), I had initially removed the tembrens, but almost immediately reinstalled them since without the tembrens I found there was an increase in side to side sway. The tembrens are now resting mostly unengaged...
Best Regards,
3 tons