I'm not sure how to adjust it, but I had a 94' f150 that did that. Until i cold afford to fix it, i tied rope to the lever on the tranny and to the hitch. When i needed to put it in park, i would engage the parking brake, and get out and pull the rope...lol pretty ghetto, but worked. Eventually I put a new shifter cable on it and it ran fine.
2000 Mustang GT-Former giggle gas addict
1990 Mustang GT w/ Vortech V3
***resident gun idiot***