Not sure on the newer cars, but it used to be that when you shifted into park a pawl engaged in the trans that kept the driveshaft from turning, thereby keeping the car from rolling. The problem is that under a really heavy load (like parking on a steep incline without putting the brake on first) the pawl becomes pinched between the two parts that it's intended to keep from moving. So much so that you cannot move the shift lever to get it out. The easiest fix for it is to get someone to give you a slight tow up the ramp while you are pulling on the shift lever.. and by slight, I mean about 1 inch.... just enough to take the pressure off the shift pawl so it can disengage.
Since it's working now, I don't think you damaged the tranny.. but I'll bet dollars to donuts that you've bent the shift mechanism somewhere. 
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1987 V20 w/1987 150HP Yamaha on a Shoreland'r Trailer
1978 16.5 Airslot w/1996 120HP Force on a Four Winns trailer
1996 V21 w/1993 200HP Mercury on a Shoreline Trailer
All towed by a 5.7L Hemi Durango.
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