'90 Lumina Issues

I have a 1990 chevy Lumina 3.1L sedan. and recently its been having a lot of problems. like rough engine. tranny issues. well i replaced some fuel injectors. and the engine runs better. but it still shakes and is kind of rough. and the tranny use to shift back into 1st very hard. now its a lot smoother. but i still cant past like 45. i can get up to 40 kind of 45. but then the car just shakes if i try to go faster. so what i do is slowly give it more gas. and i can make it to 50 easy. just takes longer then normal cars. any ideas. i was thinkin replace the ignition coils. and module. and do a tranny service. just new fluid and filter. not a complete flush though

Check fuel pressure at the rail. It sounds like you have either a fuel pump or fuel pressure regulator problem. Pressure should be 31-34 psi, key on engine off, and should raise slightly with the engine running and as RPM increases. Check for raw fuel in the vacuum supply line to the fuel pressure regulator, as this would indicate a leaky diaphragm.

i'll check the pressure. although i dont have any way to check it. but i'll somehow find out. would that cause the same issues with the tranny i described. or is that strictly a tranny issue. seperate from engine?

Your trans has a vacuum modulator, that uses engine vacuum to tell the trans when to upshift/downshift. When the engine isn't running properly, it can, and usually will, cause shifting problems also.