Is this the same in all forward and reverse gears? Did it lose fluid? If the transmission is slipping at this point, it is going to require some internal repairs. Check also that there is not transmission fluid in the coolant reservoir, or that thecoolant is not low and there have been no signs of a coolant leak. The trans oil cooler is in the radiator, and if it fails it can push trans fluid in the cooling system, or coolant can enter the trans. Eventually, if this goes unnoticed, it can cause trans problems as the coolant delaminates clutch frictions. Trans oil in the cooling system is equally as detrimental, as the transmission would obviously be low on fluid, that is not the real issue. Oil in the cooling system damages all the rubber components causing them to swell, get soft, and possibly lead to rupture. To get this oil out of the cooling system is an involved process of flushing the system, and replacing all the rubber components.