(FYI - OP's profile indicates he has the LWB, so he's most likely referring to the liftgate spoiler)
When I got mine, someone had used wire to reattach the plastic to the bracket on the one side (which, of course, I didn't notice until the wind nearly blew off the panel, at speed).
A plastic weld might (?) help; as I recall, you don't have a lot of 'meat' to work with (either in height of the post, or in diameter). I wound up replacing mine with an entire rear window assembly that I got used, with the spoiler intact -- it was actually cheaper, and my '03 had a black-colored spoiler to start with, so I didn't have to worry about color matching the replacement (IOW, I think they were all black, at least for the 1st year or two).
One word of advice, since the OP does get some snow on occasion...
Never, EVER use the liftgate with any snow / ice on the roof around the spoiler area. Or you'll be repairing / replacing it again. The larger space underneath the spoiler allows snow / ice to accumulate there (or melt into there), and when you use the liftgate / open the hatch window... CRRRAAAKKK
The SWB trucks have a different spoiler, which fastens differently / is more stable than the LWB version. So the problem tends to manifest itself almost universally in the EXT / XLs. And, no, the SWB spoiler assembly does not retrofit to the LWB. So parts scarcity can be an issue, since they were only sold between '03-'06 MY.
When the LWB were more common, I used to see people running them around here with no spoiler, and exposed bracket. One enterprising soul had rigged up a replacement stop lamp (CHMSL) without the spoiler enclosure, and had actually done a decent job integrating it. But I couldn't get an extended close-up look at it to see how he did it.