For OP and Venturamann, your issue is your TQ converter. Period.
A replacement converter "CAN" fix the problem however for a converter to lock up usually means metal shavings are blocking fluid from returning to the trans, keeping it engaged. Which gives the feel of a manual car with a frozen clutch.
The issue is that this contamination might be inside of the passages of the transmission and either the new converter will do the same upon starting or "work" for a bit until it locks the next converter.
Either way transmission needs to come off in both of y'all's
scenario.