Hemi 5.7 MDS Owner needs advice on Dual Exhaust - Dodge Man Help?

[Hemi 5.7 MDS Owner]

I have a 2008 Dodge Ram 1500 Hemi 5.7 MDS and recently had a custom duel exhust installed by a local muffler shop. The shop ower talked me into puting true duels exhaust from the cats back, meaning from each cat back to a MagnaFlow muffler 2 in and 2 out and then duals out the back. I noticed that on the highway between 60 -70 mph the engine constantly shifts that is a uncorftable to drive... also I didn't notice any increase in HP or performance. Do I need to put the y pipe, 2 into 1 back on the vehicile to provide the correct back pressure for the engine and computer system to the work correctly? Please let me know best way to go for dual exhaust that we make my Hemi 5.7 MDS perform better then the stock system without voiding the warranty?