Okay. Finally got around to implementing my crude manual shift control (schematic in thread above). Decided to go with only a toggle for the converter lock-up. Mounted the box w/switches on the front of my upper console, where I can comfortably reach over the edge to flip the switches with my arm resting on the console.
The switch nearest the driver is the converter lock-up: down connects the converter selenoid to the ECU, up sends 12v to lock the converter.
The next switch selects ECU (down) or manual (up) control, as well as sending 12v to selenoid #1 only for 1st gear when in up position. The other three switches progressively select 2nd, 3rd, and 4th as you switch them up (as long as the first switch remains in the up position).
The downside is the ECU in my 2001 Taco almost immediately throws a code when you use either the manual shift control or the converter lockup control. No big deal as long as you continue to shift manually, but if you flip the switches back to ECU mode, the ECU stays in "fail safe" mode and won't shift automatically or lock the converter until you re-set the codes. I have a semi-permanently installed ScanGauge II, so can do this on the fly, but would be a major PITA if you didn't.
The upside is that the system works great with the trailer behind, as I can force 4th gear and lockup under significant load (where the ECU would always down-shift and unlock the tranny -- causing major heat issues). Driving with OD off (as suggested in the owner's manual) results in about 3000rpm at 60 mph -- effectively limiting top speed. With manual control (4th and locked), I was able to run 70+, and get comparable gas mileage to when I used to run 55-60 in 3rd.
Here is my rig, so you can get an idea of what I'm putting behind my Taco. I've got a S/C, 7th Inj, TRD Headers, and TRD Cat-Back, so the size of my trailer is less an issue than it would be for a stock truck.