I have a 98 Chevy blazer the trans fluid has been leaking out of the front passenger side I've been filling it up now it has trouble shifting do I need a new trans or just replace the radiator the lines tranny filter and flush?

 

Check the fittings going into and out of the radiator. Sounds to me like that is where your leak is coming from.

If you have oil in the coolant, then that is a different problem and you will need to replace the radiator.

 

the fitting part number for Dorman is 800-605. They are cheap and easy to replace.