1997 Chevy Van Shifting from 1 st to 2nd

I recently towed a small camping trailer, about (4000 lbs). All was fine till my return trip, when I noticed the tranmission would not shift from 1st to 2nd unless I lifted my foot monentarily from the accelerator. The fluid level looks OK, and I had already installed a tranmission cooler. No change after I got home and unhooked from the trailer. It winds the engine to 3500 rpm or more, then slams into 2nd gear, or if I lift, it shifts pretty good. Do I have a sticky solenoid, or something worse?
Thanks.