check engine light and reverse gear

I have a 1996 Chrysler Plymouth Grand Voyageur with about 130,000 miles on it. The transmission was replaced about 3 years ago, somewhere around 90,000 miles--just before I bought it. Recently after about a 30 minute drive, I put it in reverse and the check engine light came on and the reverse gear works but it takes alot of power to get it to back up---you can stomp on the gas but it still backs up really slow. Putting it into drive works perfect, what could the problem be??