I have a 1989 Jeep Wrangler Laredo 4.2 liter automatic?

It's hard to say without phyhsically looking at it, but it doesn't sound like your fuel injection kit. It sounds like your throttle position sensor or your Idle air control valve. I'd lean towards your TPS because if you took it off-roading and got any water suck into their or something it can damage the sensor. It could also be something happened with the injection kit while off-roading. It's hard to tell. any check engine lights?