The other day when I was driving, I came out of my neighborhood going up hill. I was stepped on the gas a little bit to make it up the hill and it slipped and gave out. Luckily I was down the street so i reversed home and checked the fluid, it was somewhat low, considering I live on a small slant. After filling it up and letting it run for a few minutes I tried again in Drive and it just revved the engine. I tried 1st gear and it did the same, then 2nd worked. I drove it around my block and shifted it into Drive after getting to about 2k Rpm and it shifted right into 3rd and then into 4th like nothing was wrong. Coming to a stop it automatically shifted into 1st and again it just revved so I just drove it home in 2nd. Any ideas on what happen? And I recently had my rear drive shaft re-balanced.