My name is Sean, my03 Silverado 5.3 has 245000 miles on it and within the last week has starting shifting really hard only when it has reached operating temp, it has thrown 2 new codes P0894 and P0420, not sure how they are related or what my issue might be. Please any ideas help.

The p0420 a cat efficiency fault and p0894 is trans component slipping. The trans slipping issue will default in high line psi to try to save the trans from a slipping component. Is the fluid level full and can you smell a burned clutch in the fluid(similar to over heated brakes). If ok you can watch for excessive TCC slip. Hope you find this helpful.

You are kind of stuck between a rock and hard place.....

With the mileage you have, you are in store for some expensive repairs......replacement of the catalytic converter(*), and possible overhaul of your transmission........checking Bluebook value, not worth it based on 2 wheel drive, LS Model, 8' bed, standard options...