whats wrong with my truck

i have a 1999 ford-150 with 35k miles. it is 2wd, 4.2L, and regular cab. I recently went in some dirt fields with it and had fun. when i was done the "service engine soon" light was on. now the exhaust is louder, the truck vibrates when i am just idling. I have very little accerlation, power, and torque in my 1st and 2nd gear. also the truck sometimes vibrates around 50mph. I don't know it this is it but sometimes it feels like something is slipping possibly gears. thanks for any help. I want to know what could be wrong with it before i get charged at a mechanic.