Very odd that U-Haul won't approve the Tundra for towing the 4-Runner. I tried a couple of different tow vehicles on the U-Haul website and even a 2018 Tundra with a 10,000 lb rated hitch is not acceptable. Could be the weight of the towing vehicle vis a vis the towed vehicle, although it does seem arbitrary. U-Haul should spell out the criteria for the towing vehicle.
Regarding rates, both with Penske and U-Haul, season matters: summer is peak moving season, therefore peak truck rental season. And origin/destination matter. Demand in each area of the country is a significant factor in determining rates. If a city has significantly greater outgoing rentals than incoming rentals, the outgoing rental rates will reflect that. Although, odd things happen. In November I rented a Penske 16 foot truck from Rockville, MD to Prescott, AZ and did the same last month and the rate for the same size truck, same origin/destination was $500 less in July than last November. Go figure.
Good luck!