Car won't accelerate in drive

Ok so here is my issue,

I drive a 96 cavalier (has about 180k miles), got it about 3 years back from a police auction. It had some minor issues which we got fixed ASAP as far as a muffle,brakes etc.

Just this past April i sort of had a breakdown (bad spark plugs) and got that fixed as well.

Now I'm having an issue with the transmission I think. The car reverses fine and goes in all the gears,however, when I put it in drive it refuses to go.

I tried putting it in over drive (1,2,3) and it went fine in those so I have no clue whats wrong. I don't smell any burning or anything only some exhaust fumes (since I just found out my muffler has a hole in it) so i don't know what to do.

I talked to some people and they said it sounds like the transmission fluid needs to be flushed/changed. When I first got the car I believe it had about 160-165k on it and I'm not sure how the person before me took care of it.

I thought about just getting an entirely new transmission, but if I had a faulty transmission it wouldn't go at all right?

Any help would be greatly appreciated!