Plugs

ok here is the story. I bought a Ls tranny from a friend for $150. The reason I got it so cheep was because, My friend was doing Dounuts in Reverse and the shifter Pop out of gear. At first he couldnt get the tranny in any gear. Then it worked its way into 1st and went threw 1st-5th with NO problems. BUT. It wont go into Reverse AT ALL. Could anyone Tell me How to fix this problem Or could you tell me what I need to look for on having this problem Fix?
And If I need a New reverse gear how much are they and do any of you have this for sale?