I have a 1993 Chevy Silverado K2500 6.5 L Turbo Diesel with a four speed automatic 4L60E. The transmission has 1st, 2nd, and 3rd gears which shift one to the next flawlessly...that is, until it gets to 4th. When the problem first started, the overdrive (4th gear) was spotty. Sometimes it would slip back down to 3rd and you could put it in neutral and restart the truck and it would work fine. Then one of the heads on the motor cracked and I rebuilt it. After the rebuild the transmission would shift into O/D and then downshift to 3rd immedetly after and would not shift back into O/D until you completley stopped, put it in park, restarted it, then took off again, even then it would do the exact same thing over and over again. Then one day I took it for a drive and it didn't even try to shift in to O/D and it hasn't since. Then the motor was making a terrible knocking sound, so we rebuilt it again, and the same thing, no O/D. I let a technical school rebuild it (for the experience) and they messed the shift linkage up, so when it says its in neutral (N), then its acutally in O/D. Now heres the kicker, when you downshift to just D from O/D, the engine brakes come on and RPM's go up by about 500 the truck starts to slow down. And when you upshift back to O/D, the engine brakes shut off. But still no O/D. This is a heavy duty hauling truck, therefore it has crossover gears (gears in between gears so it can downshift when going up a hill and such without actually fully going back to 3rd gear). This tells me that its getting stuck on the crossover gear from 3rd to 4th. What could cause this? I know the trannys not goin out because it's been driven like this for 20,000 miles and the first 3 gears work flawlessly still. And I replaced the brake switch and cleared all the codes so I know its not those two things causing it. And I smelt the transmission fluid and it doesn't smell burnt at all so I know it's not a burned up band. All help is welcome and greatly apprechiated. Thanks .