The way to be sure that the lowering solenoid isn't being commanded on is to disconnect it at the solenoid. Not sure about your exact model but most I've worked on have a black plug on the lowering solenoid that's held on by a screw in the middle of the plug. Just disconnect.

To check the lift cylinder, put a gate valve in between the pump and the lift hose. Lift up and then close the valve. If it continues to lower then you've got bad packing. If it comes to a stop, then the lift cylinder is doing its job. The lift cylinder also probably has a blow-by return tube made of clear 1/4 in plastic. If you see a steady stream of oil going through it as you lift that would also indicate bad lift cylinder packing.

The solenoid valve can be stuck open by debris in the valve. Sometimes you can fix it by cleaning. But watch out for springs as you disassemble.