I had gotten a new washer. The main line is 2" and runs under the slab. The 2" line comes up 12" and then T off too a 1 1/2" line with a P trap. The 2" runs straight up for vent pipe.
When the washer discharges it backs up the 2" vent that is is 8' AFF and around the 1 1/2"
I have snaked the 2" line and still have trouble. What is my next step?

Today's washing machines have powerful pumps that discharge water too quickly for old sewer pipes. Your washer should have 2" P- trap attached to 2" Sanitary "T" and at least 30" of stand pipe. Unless you have this set up, your washer will continue to overflow.

I had gotten a new washer. The main line is 2" and runs under the slab. The 2" line comes up 12" and then T off too a 1 1/2" line with a P trap. The 2" runs straight up for vent pipe.
When the washer discharges it backs up the 2" vent that is is 8' AFF and around the 1 1/2"
I have snaked the 2" line and still have trouble. What is my next step?
We have had quite a few complaints like yours, especially with new MayTag washers) where the washer pump is too powerful for a 1 1/2" trap and stand pipe. I would increase the trap and standpipe to 2" and install a compression fitting,(see image) on it.
I have a solution that I've never had a complaint on and one that I had to use on my washer. You close off the stand pipe the washer connects to with a compression fitting,(see image) or I caulked the hose in with rags and sealed it with duct tape. This makes a water tight connection between the washer and the stand piipe. Nothing can leak out or overflow and the force from the pump will send the discharge out into the main drain. While there is some back pressure involved there will not be enough to damage the washer pump. Outside of oversizing the washer trap and standpipe this is about the only solution that I know that works. Good luck, Tom