87 camaro, 700r4. Tranny has whined on decelleration for some time, over a year. Recently it has gotten problematic. If first gear shifts at 5500rpm, the rpms will hang at 4500 for a few seconds before falling. After an hour of freeway driving to get back to school, the car started making clanging and grinding sounds on decelleration at high and low speeds. I've never heard anything like it, but it definitely sounds like something is amiss in there. Tranny fluid and engine oil levels are both fine. Power on the engine is at least 100 horsepower over the stock 190. I have never touched the tranny, and the fluid has not been changed in over a year. Fluid comes up to perfect level on the dipstick, red in color. 135k miles on the tranny.

What's wrong? What will this cost to fix? Should I just purchase a 5/6 speed to replace it?