The torque converter is a viscous (fluid) coupling with a lock up - while viscous it uses the transmission fluid for the engine side to turn the transmission side (like a fan blowing a second fan and causing the second fan to spin) - they can and do then spin at different torque levels (ie engine turning 3000 rpm transmission input turning 2200 RPM = 800 stall, or engine idling at 950RPM transmission input 0 = 950 or higher stall). When under slight loads cruising or at RPMs above 3500 you get WAY better economy if both sides spin the same - mechanically locked together...
A bad solenoid, programming OR convertor can try to lock and its like dumping the clutch in a stick...
J Wm Bishop EA, ASADE
Money cant buy happiness. But it can buy Jeep parts. Same thing.