Sputter when low speed acceleration and at highway speed

I live in the Midwest (MN) and I have the truck listed above FX4. Twice now in the last 2 months I have noticed that while slowly accelerating, key word slowly (not stepping to get up to speed quickly) but your average pressure on the gas pedal that would be used when just starting out and accelerating in town type of driving slowly, that they engine will seem to misfire/spudder. It is not a constant misfire/spudder but periodic). The tach will slightly drop 500 RPMs from 2,000 to 1,500 for a split second like it didn't have any fuel or lost power and then kick right back to normal tachometer/power level. The lights do not dim nor any noticeable loss of electrical power, other than the tach slightly dropping causing a lurch or studder in movement. There are no codes/lights being thrown. This will continue off and on while driving. If I am stepping on the gas harder forcing more fuel in it does not happen while accelerating harder. At least I do not feel it if it is. The plugs, and I believe the wires were changed out right before I purchased the truck 2 1/2 years and 20,000 miles ago-By a shop due to one of the plugs snapping when the prior owner was servicing it). The 1st time this happened in late Nov after a 2-week 3,000 mile hunting trip it lasted about a week (drove twice after getting back). I brought it in for an oil change and planned to bring it to my local shop from there, but it stopped doing it on my way to the oil change. I figured it had bad gas. Anyhow, I topped off the tank and no issues after the oil change for almost 5 weeks. I went on another trip in Dec for Christmas. Put another 1,700 miles on it with multiple tanks of gas. After getting back the truck sat for about 10 days with a half tank of gas. We had some snow so when I got back in to take to work one day the sputter was back. I filled the tank, again was at a half tank at the time and no go. Still doing the low / Cruising speed RPM shudder. Sorry it's long but I wanted to provide as much detail as I could. Ideas? With no codes, I figured to start with the cheapest possibility (fuel filter) and work to the more expensive. Any thoughts or have experience with this? I did also just have the rear end replaced w a lower mile rear end in late Oct due to the pinion bearing going out)-although I would guess that that does not play a role :) - Thanks for any advice you have to offer.