Really hate to say I agree with who ever told you replacement, but I am afraid that is the case. Because of the miles on the unit, you will be hard pressed to just repair it. The reverse reaction sun shell has come apart on it. Pretty common thing on these transmissions. When it happens it can take other gears with it, or just the reverse or just one of the other, or a combination of them. . I will post a copy of a tech bulletin released by GM that covers this problem for you below. They have come out with a new stronger replacement for it. But, that does not help your situation. Because of the difference in prices for labor from one area of the country to another, and because of how often prices change in todays economy, I am even hard pressed to give you a price. But the trans will need to come out and be torn down just to replace this part. Then you have already invested as much in labor as you would to have the unit rebuilt or real close. Only difference being what parts get replaced. You can shop around for a new trans, a rebuilt unit or even a used one. Difference being price and labor to install. With close to 300,000 miles on it you would be hard pressed to find anyone who would be willing to just replace the reaction carrier in the trans, because the clutches and many other parts are going to be so worn at this mileage that no one will be willing to stand behind the repair. The best and fastest would be a used trans, but then you have to hope you bought a good one. If you check with local GM dealers, many times you can get what is called a SERTA, which is a re manufactured unit from GM with the same or sometimes an even better warranty than a new one. The labor to install is going to be about the same if you go used, re man or new. With aXXXXXyour looking at 6 to 8 hours labor at what ever the going rate in your area is, just to replace the thing. A good used one will run you a grand or close, depending on the miles on it, etc and a re man around 1500, new closer to 2500. To install either a re man or new is going to run you between 2500 & 3000 out the door. Even a use one will be a grand to 1500 out the door. And I have to feel it would be that much if you found someone who would just replace the sun reaction shell. I know this is not what you wanted to hear, but there is no good way to deliver bad news that I am aware of. I will post the bulletin below for you. Wishing you luck, which ever way you decide to go. Thanks
This bulletin is being revised to add 2005 model year to the parts information. Please discard Corporate Bulletin Number 00-07-30-022C (Section 07 -- Transmission/Transaxle).
Some customers may comment on a no reverse, no second or no fourth gear condition. First and third gears will operate properly.
The reaction sun gear (673) may not hold inside the reaction sun shell (670).
When servicing the transmission as a result of this condition, the transmission oil cooler and lines MUST be flushed. Refer to Corporate Bulletin Number 02-07-30-052.
Follow the service procedure below for diagnosis and correction of the no reverse, no second, no forth condition.
Important: If metallic debris is found on the transmission magnet, the transmission must be completely disassembled and cleaned. Metallic debris is defined as broken parts and pieces of internal transmission components. This should not be confused with typical "normal" fine particles found on all transmission magnets. Failure to properly clean the transmission case and internal components may lead to additional repeat repairs.
Hate to say it but no. Won't work. The 4L60E and the 4L80E are similar and these trucks came with both units. Problem being the changes in electronics between 96 and 2000. They are programed differently and the solenoids and internal electronic components are not the same. I don't think the wiring harness connectors will even interchange. The did make a 4L60 and a 4L80 that did not have the "E" on the end. These were not electronic. So there are some differences. A non electronic won't shift right, etc. in a truck set up for an electronic. Some info to help you with identification posted below. Thanks
Transmission Identification Information