GW the lates - and also the paid repo. Chrysler (Chase) removed a paid repo my sister had after 4 years. Chances are you won't receive replies to all your GW's - but keep trying every 30-45 days. Never give up. Are all your lates of the 30-day type? Work on the most recent lates first.
I would suggest you might want to consider getting a store card and a gas card to improve your credit mix. I have 4 CC's, 2 AMEX cards, a store card and a gas card. I keep small balances on 2 cc's (<5%). The other two have zero balances.
Continue paying on time, that and the natural aging of your cards will move you up.