I recently bought a 1992 Jimmy with a 4.3 Vortec, and 199k miles. The transmission seems to work fine, except it wont downshift very well into passing gear. If i floor it, the RPM's go up a little, and it does speed up a little, but i have to hold it down for sometimes 7-10 seconds before it'll downshift. By the time the passing gear kicks in, i have lost the chance to pass. Any help would be appreciated.

Is the TV Cable hooked up and adjusted good?
Engine running good?
On a normal take off what MPH does it up shift from 1-2, 2-3, 3-4 and lockup?

I saw a bulletin on something like this. S/T 4.3 [LB4], poor performance, no downshift.

The diagnosis for this was to check the TPS signal at idle, should be under a volt about .6, then kill the engine, turn the key on, you don't have to start it. Have a helper press the accelerator to the floor, check the tps signal with the pedal on the floor, it should read about 4.6v, if it doesn't, then the bulletin says to replace the accelerator pedal assembly with a revised assembly. Then you do the test again, if the voltage still isn't correct, then you replace the accelerator cable assembly with a revised assembly.

The point of the bulletin, your throttle valve may not be going WOT. After testing the voltage on the TPS, with everything hooked-up. You might take the throttle cable loose from throttle valve, then open the throttle up by hand to see if the readings improve.

Might not be your problem, just a thought.