I bought a 300m that the tranny works fine in forward gears but when you put it in reverse it acts like its in park. When you put it in park it acts like its in neutral. Ive checked the cable and it seems ok but im not sure. Ive been told there is a solenoid that is bad but I do not know anything about it. Im open to any ideas. Has anyone experienced this before?

The first course of action should be to have it scanned to see if there are transmission related codes.
Without that information, there's NO way to start tracking the problems down
Yes....it could be a solenoid, relay, or even a hydraulic issue.....but you have to start with a scan.

Some newer Mopar you can turn the key switch off & on 3 times(not starting it) & the light will flash on the dash if their is any, like flash 2 times then stop a sec., then 3 times for code 23, I have not done that yet wear I have no newer Mopars, but you may be able to try that on yours & see if you get a code!