This happened to me as well...
I took the car to an independant transmission mechanic, it cost about $800 (Australian dollars) to fix. No need to replace the whole transmission, just something specific to reverse gear. The prices that were quoted by several transmission shops varied between $5000 and $800.(AUD)
That was about 18 months ago, car is still running well.
While it's in being fixed, it's a good idea to replace the tailshaft flex discs as well.
good luck.!