Put on a new muffler (V Force which is comparable to Flowmaster). Ever since, it seems to mess around with the transmission a bit. Every now and then, when barely giving it gas at lower RPMs, the RPMs will now go up and down by about 100RPM rapidly. I've also noticed two hard shifts, sort of where the truck couldn't make up it's mind whether to up shift or down shift, so it did both in about a second, and felt violent. It's got something to do with sensors not sensing the appropriate backpressure I'm betting. Someone with experience/knowledge chime in please. What do I do? For now, I'm driving in Tow Haul mode and it seems to help the truck figure out what it's supposed to do, rather than the sensor kicking in and acting a fool....