2000 Other Jeep Models Irregular Rev'ing

2000 Jeep Grand Cherokee Laredo with close to 120,000 miles on it. When I am cruising along at 55 mph you can feel and hear the engine rev. The tach will bounce back and forth between 1700 rpm and 2200 rpm. I tried to go 60 to make sure I wasn't at the threshold of overdrive kicking it, but it still did it. Is this the transmission? I was just told I had a misfire on one of my cylinders and my sparkplugs were replaced. Will this fix the problem. I also had a fuel injector code on one of the cylinders, but the shop said that cylinder checked out okay.

Thanks.