FYI, here is a link to the adjustable time delay switch I purchased for $40 from Baker Electronix. It is probably cheaper to build one, but this is contained in a very small black box with a small control knob to adjust the length of the delay. You can specify the range of available delay when ordering.
Baker Electronix - Time Delay Modules
Can you put the time delay switch/circuit between the 4th gear switch and the TC solenoid so that the lockup doesn't occur immediately?