Welcom to GMSB,
There is a connector on the drivers side of the transmission, make sure thats connected.4 pin plug with 3 wires is in tact, and connected. 1-Yellow wire, 1-Blue wire and 1- Green wire
You should also have a brake switch unfer the dash Brake pedal has 2- switches on it. One controls my Brake lights (orange & white wire) the other I assume is to the Torque Converter (1-Gray wire & 1- Blue wire I think??) Wires do not look the same guage or color as the ones coming from the plug on the tranny though...., with that same wire, make sure thats connected securely, and is adjusted properly Not sure what the propper adjustment is on this, but I put a test light to it, and when the brake pedal is depressed the Gray wire looses power. When the brake is in the unpressed position it has power. So once again looks like all of this is functioning properly???, this disconnects power fro lockup when you hi the brakes.
Under the hood, on the firewall, ther should be a vacuum switch, Found this also it is over near the heater/AC box. Have not checked it for 12V yet but I did replace the Vaccum line that went to the back of the Intake Manifold.....with 2 wires, on comming from key power, and one going to the brake switch. trurn the key on, and confirm that your getting 12 volts on one of those wires. I need to confirm this.
while your at it, amke sure that the vacuum line is going to a thermal vac switch on the intake manifold, and also make sure the switch is connected to a manifold vacuum source. Not sure what a thermal vaccum switch is?? It was connected directly to the intake manifold
If this all checks out, powered, and connected, and still no worky, i would get a 12 volt switch, like from a set of fog lights, and a couple of lengths of wire, wire one end to the 12 volt side of the vacuum switch, and the other side to the wire in the brake switch plug, going to the transmission. then drive the truck, let it get to around 40 mph, and flip the switch, and see if you notice lockup, if so, tha vacuum switch needs to be replaced. if not, then the next step would be to open the trans pan, and replace the TCC lockup solenoid, its right behind the bellhousing, in the valve body, held in with 2 bolts. replace the trans filter while your at it. and if that doesent work, either you have a bad clutch in the torque converter and/or 4th gear is totally gone.
A lot of work, yes, but you can save gobbs of money by doing these steps yourself. if it turns out that the trans is bad, I would recommend going to the wrecking yard, or craigslist, and find an 88-92 700r4(some refer to it as the 4l60) and have it rebuilt at a good shop, who should be able to do a good rebuild, with a new converter for around $700. Its a way better unit than the earlier ones.