Do I have a transmission problem?

I own a 1989 Dodge D150 pickup with a 3.9 V6, automatic transmission . The truck has been sitting for over a year (long story). The transmission is shifting hard in all gears. It isn't grinding or doing anything else unusual, just shifting very hard. I'm wondering if this could be a result of having sat for so long and just driving it a little more will make it stop or if I have a serious transmission problem.