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Yes it can be done. You'll need several sizes of metal drill bits, a sander/metal file, and some duct tape.
This is a hard job, but it's much easier if you can stick the transmission in a vice to hold it still while you drill. Once the case is drilled, pull the output shaft and insert the new gear onto the end of it. Give it a few taps with a hammer to seat it properly. Then slip the input shaft back into the transmission.
Now, some people will say that you need some sort of cap for the transmission where you drilled so the fluid won't leak out, but I disagree. Several layers of duct tape works just fine. When the oil starts to lubricate your new gear and leak out, it will obviously stick to the duct tape, leaking problem solved.
Let me know if you have trouble and I can try and walk you through it.
Why would you want to, anyhow? Fourth gear in a 700R4/4L60/4L60-E/4L80-E is .69 to 1, anyhow... there's only so far you can go with overdrive gears before you bog the engine down to where it's counterproductive. Not to mention overdrive is an additional parasitic drain placed on the engine, which is why I'd rather see companies use shorter rear ends and four/five/six speed direct drive transmissions.
What you actually *could* do, (probably) is put in a Richmond Gear 5 or 6 speed manual tranny. You would have to hang a clutch pedal, and no doubt some fabrication of mounts and brackets would be required.
I'm not certain this would be worth the effort, in fact I sort of doubt it.
What are you trying to accomplish OP? Better MPG, better performance, combination, just want to do something different?
Your truck is pretty heavy and not very aerodynamic, so I am going to guess that the factory gearing is pretty close to as tall as you can run and still have a practical truck. At least with the stock engine.
I seen external overdrive units for manual trans, so do they have this for this trans as well? I am sure with enough money, you can get a shop to do it. I have no idea why you would do it, the R4 as a poster mentioned already has an overdrive gear, and even if it did not, there is no way this would be saving any money via mpg if that is your goal. Your engine probably does not even have the power to push a higher OD ratio.
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