2001 Lincoln Navigator Transmission or Rear End?

Hello and thank you for reading this post. I have a 2001 Lincoln Navigator 5.8L, Automatic, 4WD. When I come to a stop, it feels as if I have been rear ended. There is a loud "clunk" sound at the rear of the vehicle. Then, when I accelerate from that stop, the engine gets up to about 3500RPM before it SLAMS into 2nd gear Everything is fine as long as I keep going. Transmission or rear end or both? But once I slow to a stop, it starts all over again. Fluid levels good and clean. No leaks. I would greatly appreciate any guidance you can offer.