You need to find out if it's the shifter, the cable or the tranny......
You will need another person for this......you will have to pop the cable off the shift linkage of the tranny.....start car with hood open....keep foot on brake and have someone manually move the shift linkage on the tranny, one detent, to reverse, back to park, and back to reverse and see if the vehicle moves.....if it does, but you have problems with the cable connected, you know it's not the tranny....
By the way, I ASSUME you have checked fluid level and condition?