Engine revs up with barely any acceleration

For the most part the car hasn't had many mechanical problems but recently and slowly lost power to me rear axle. Thought it was a transmission problem because the clutch was engaging fine, the engine was revving up fine, there was coolant keeping the car cool. But every time I'd step on the pedal the car would barely move and the rpm's would shoot up. Had to tow it home and checked the transmission fluid in the drive way. Gear oil looked a little dirty but didn't find any metal or seemed to be low on oil. I'm not sure if it's actually the transmission but I am getting tire movement it's just over working the engine.