'98 GMC Jimmy 4 Door, 4x4

When driving with the engine under a sustained load (e.g. driving uphill at around 2,400 RPM 55-60mph) I will lose and gain ~ 50 RPM every few seconds, alternating. I've had it happen with as little as 5 seconds between change, to 15+ seconds between change. Happens pretty consistently.

Up, down, up, down, up, down, etc. Noticeable change in engine pitch, and I can feel the loss/gain in the truck itself. This doesn't happen when I'm just cruising down the highway.