1999 Chevy Suburban Major vibration pulling hills

Within the last few months, we've experienced an intermittent major shudder/vibration, primarily when pulling hills. Traveling at interstate speeds (50-60 mph), as the suburban starts up the hill, it struggles to maintain power and speed begins to drop. The whole car shakes violently under the load and rarely gears down for more power. Even if the transmission does gear down, that lower gear isn't generating enough power to accelerate up the hill, and the vibration continues. For the record, we primarily drive in 2WD.

The 'check engine' light came on during one episode, and the code indicated 'random misfire'. So, we had a complete tune-up, since the mileage was approaching 100K. However, it's experiencing the same symptoms.

We've also changed the fuel filter, serviced the transmission and added a transmission additive to improve performance of the torque converter. Nothing has helped, and since the check engine light no longer illuminates under these conditions, there are no codes to help.

Could it be something in the valve body? Is this a common problem with this generation Suburban?

Since receiving the engine tune-up, it occasionally idles rough when warm. Any connection? Ideas of what to try next?

Thanks!