Just bought the Truck last September and was very smooth in all ways until in December when I re-tuned with EFI plus trany tune and at same time installed the 2 micron "Cat" fuel filter.
I sensed a pulsing right away which is tolerable running alone.
However when towing it becomes far more apparent/exaggerated and very uncomfortable especially when towing in the mountains because up every grade it creates chucking all the way.
With the initial EFI tune it did not do this when towing.

Like I said our first two months of experience was great. My installer has scanned while towing and finds nothing amiss.
Other than the retune the only difference compared to September is much colder weather.

We are at a loss as to why this is happening. HELP?