Most likely it's the T350, mine is very hard to move when messing with tires and brakes or trying to push the truck out of the garage. Depending on the gear ratio, you could be trying to turn the pinion quite fast which will make for a tough turn of the wheels. Third member shouldn't have any effect if both tires are off the ground. Pull one drum off while adjusting the other and having just the tire and wheel on will help rotate due to the extra weight and spinning mass.
