The torque converter lockup is controlled by the TC clutch control valve located in the pan of the trans. GM started using this valve in 1982 ( I think), when they introduced the TH200C trans. As near as I can tell, they have never changed the design since. It is used in the TH200C, TH200-4R, TH700-R4, and the later 4L60 and 4L60E. I've had trouble with these valves in every vehicle that I've owned that uses it, except for one. The newer transmissions do seem a little more trouble free.
The first thing I would do, would be to change the fluid and filter. If the fluid is dark, or burned, then I'd change the fluid 2-3 times until it's lighter; no need to change the filter after the first time.
The valve can be replaced in under two hours, including the time to drop the pan and refill the trans. It's bolted into the front of the valve body, with 2 or 3 bolts, and an electrical connector. The valve is about $25-30, the last time I checked and is available aftermarket.
I don't think the valve is controlled by the ECM on the '93--it is on later ones. Earlier vehicles had a 4 prong plug that connected it to the trans. I'm not sure about yours though. There is probably a connector on the brake pedal that opens the converter when the brakes are applied, and it's probably part of the brake light switch. Removing the correct wire here should disable it.
GM does not recommend that you disable it, however, as it's theoretically possible to overheat the converter in 4th gear with it disconnected.
