2002 Toyota Tundra transmission problems

While driving on the freeway my truck made a bad sound from the transmission a gear grind clunk noise. I pulled off the freeway asap and put it in park. There was no fluid draining or any thing. When I went to put it in drive it was hard to go into gear. But it drove ok so I went to my appointment less than a mile away. From there my wife picked me up and we did some christmas stuff and got back to the car later than expected and I had to drive it home (aprox 5-7 miles).

2002 toyota tundra 4x2 sr5 with under 18,000 miles, at 45 degree angle while in drive and no load, vehicle will roll backwards and will continue to roll until the brakes are applied. While rolling a loud constant clicking noise eminates from the rear end as if gears were slipping. Vehicle seldomly driven and almost never pulls any loads, never abused or wrecked and maintained regularly. This has occurred since the vehicle was purchased in may of 2002.