Shaking while accelerating between 50-65 mph

Hello all, looking for advice to see if I can narrow down an issue I have been noticing. My 2008 base model Prius has developed a "shaking" while accelerating between 50-65 mph. It happens during low-mid acceleration but not if you floor the gas, and it goes away when I'm cruising, or at speeds higher or lower than mentioned. I don't feel it in the steering wheel, and as noted it is not constant so I can rule out tires at this point (brand new tires on the front and the old tires did it too). The car has had most high mileage maintenance done, including new cv axles, wheel bearings, and spark plugs. My first thought is a bad motor mount. The main torque strut on the bottom of the engine looks ok. I would also say the shaking is slower and rhythmic rather than a harsh vibrating or anything like that, and that it is more of a "side to side" shake. Could this also be a transmission issue? As far as I know, the transmission fluid has never been changed but I doubt it would be related (transmission fluid shouldn't throw anything off balance). Toyota states it is good for the life of the transmission but I wonder if they didn't know it would last well above 300,000 miles. Any reason I shouldn't change it? I'd prefer not to spend $200 on a full motor mount kit if it's the ball joints or something I might not be thinking of. I'll be driving from Cleveland to Colorado soon and will be driving in the Rocky mountain a good bit so I'd like to get this fixed asap. Any ideas would be great (no I will not be taking it to a shop, I do all my maintenance myself). Thanks in advance!