Slipping when in Park

[Hooty]

Yesterday I parked my 2003 2500HD 6.6 Alison truck in a friends driveway and put the transmission in Park. I started to get out of the truck and it was slowly rolling backwards. I jambed on the brake, started it up and pulled ahead a bit, then put it in park and it did the same thing. It was rolling back very slowly as though there was a brake on, not free-wheeling. I let it roll for about 3 feet (waiting for the parking pawl to engage) and when it didn't, I stopped it with the foot brake. Applied the parking brake (I know, I should use it all the time anyway), and went in to visit my friend.
After 10 minutes I started the truck and drove home (about 3 miles) and parked in my driveway. I put the truck in park and it held fine. I have tried it a couple of times and it doesn't slip now.

Is this common? Perhaps the tip of an iceberg? Has anyone else experienced the same thing and is there something that can be done to correct it without pulling the transmission, i.e. and adjustment of some sort.

Cheers

[Mike L.]

Probably a worn cable or cable not secured in the bracket properly.