So My 04 legacy jumped a gear pulling into a parking lot threw a p0866 engine code defined as a TCM Communication Circuit High. Also the flashing AT oil temp light. Is there a diagram I can see to find out if I’m able to access the issue or is it more internal and I’ll have to pull the trans for repair 

AT oil temp light shows your tranny is overheating. Normally this happens if the tranny is low on fluid. If the tranny is really low is will slip in and out of gear.

Get under there and check for leaks and check the surrounding area's for wet spots since tranny fluid can shoot out under pressure while driving yet won't have a active drip while just sitting.

"flashing AT oil temp light" means there's a code stored in the TCM (vs. ECU/PCM). Search on here for a DIY to pull the codes.

Then you can decide the next step.