I know its sacrilegious to suggest this, but.......
Its not a Ford is it?
Fords of that era had a recall for just that problem. My '79 had it, and the sucker shifted into reverse just as I was getting out to hook up the winch, and it chased me backwards across the yard until I could get my feet under me to get back in.
For the life of me, I don't remember what the problem was. Something about the springiness of the shifter linkage that would let the column shifter go to park, and seem to lock in, but the actual shift lever on the side of the transmission did not go all the way into park.
It usually showed up when you slammed the door, but in my case I bumped something as I got out and away it went.
Well, I guess everything is on the internet. I found the original problem. Turns out, Ford never found an actual problem, or said they didn't. They just sent out stickers to advise people to be sure the vehicle was in park and to set the parking brake whenever they got out.
However, the problem does not seem to be confined to Fords. A 1976 Chevy van went into reverse and ran over a guy.
Here is the link to the report.
http://www.fordification.com/tech/auto-trans_recall.htm
I never did anything to my truck, I seem to remember it trying to do it a couple more times. I just developed the habit of putting it in park, and giving it a tug before I got out.