Chevrolet - Tahoe :: 1996 - Shuddering At About 50 - 55mph While In 4th Gear

I have a 1996 Chevy Tahoe (5.7L, 185k miles) that I think is misfiring. What typically happens is a shuddering at about 50-55mph while in 4th gear (though to a lesser degree, it will shudder at speeds above that too) and lightly accelerating or keeping speed on a small incline (~5-10% grade). During this time, there is no change on the tach. My check engine light is not on. Spark plugs and wires are new. The problem occurs while using 93 octane fuel. The shuddering is never noticeable in gears 1, 2 or 3, and is not noticeable at full throttle. I replaced the fuel pump about 6 years ago.

I brought it to the transmission shop, but was told that isn't where the problem lies. Second, I took it to a mechanic. He told me that he couldn't pin-point the cylinder, as there were no codes to unveil the problem.

My question is what are the causes I should look for, and how can I address them short of an engine overhaul? I recently read for the first time about Seafoam, and I'm intrigued.