To be absolutely sure, you can identify your transmission by these methods:
* Transmission Identification Tag: Look for a tag on the transmission case itself. This tag will usually have the transmission's part number, which will clearly indicate whether it's a 4L60E or 4L80E.
* Case Size and Appearance: While not foolproof, the 4L80E is significantly larger and physically longer than the 4L60E. A visual comparison (if you know what a 4L80E looks like) could offer a clue.
* Pan Shape: The oil pan shape is slightly different between the two. The 4L80E pan is generally deeper and wider. Again, this requires familiarity with the shapes of both pans.
* Dipstick Location: The dipstick location may be slightly different. However, this is not a reliable method for identification alone.
* VIN Decoder: While not always completely accurate for transmission specifics, running your VIN through a vehicle history report service (like Carfax or AutoCheck) might list the transmission type.
In summary: Given the engine size (4.3L) and the vehicle class (half-ton), it's extremely unlikely you have a 4L80E. Focus your efforts on finding the transmission identification tag. If you can't locate that, visual comparison is the next best option. The other methods are less reliable on their own.