95' V6 trans issues

I just bought a 95' 3.0 V6 4 runner that had overheating issues that caused head gasket to fail. I fixed all the reasons it was blowing the gasket with a new radiator & thermo. Now, temp no longer an issue but the automatic trans is slipping but has been getting alittle better as I drive it more. I noticed the trans fluid was pretty burnned when I replaced radiator..so will replacing the trans fluid & filter fix this issue ? Or could there be another issue ? My check engine light is also on, I will check the codes soon. I am getting horrible gas mileage, but probly from the slipping trans I guess. Any suggestions on a plan of attack to combat these issuses ?

Changing the fluid and filter is a good place to start, but it may not fix the problem.
It could be clutches, solenoids, etc. in the trans.