93 ranger 3 liter auto, why wont it go into gear?

auto trans. My truck stalled to day when i stopped it started back up fine then tried drive overdrive 1st 2nd and reverse and would stall. I had to rev up the engine then shift before it would go anywhere. I drove it about 10 feet before it stalled again. I know it has new alt. plugs and wires. I just filled up with gas full tank and drove it maybe 40 miles. Checked the trans fluid it seemed a bit dark but it shifts ok. plenty of power before this happened. I added some coolent because it was bone dry a couple of days ago. not sure what to make of this any ideas?????