A quick search on car-part.com DOES indicate that the rear glass (and thus, spoiler) is interchangable, as a whole host of EXT vehicles popped up on my search for 2003 (really, 2003-2006 would all fit, for the LWB models).
If you found a window w/ spoiler for $29 - grab it. Ask if they have two
(you could sell the other and make your money back for both, easily).
Just check the defroster grid to see if there are any breaks (you can feel along the inside edge and determine this. Breaks in the horizontal sections can be repaired w/o too much trouble. Breaks on the vertical ends are a little more difficult (there are posts here describing how to fix broken gridlines)
When you go to swap them out, a helper would be good to have, along with a small drift / pin punch, and a flat-blade screwdriver to loosen the strut mount retainers. Lift the glass, but leave the main gate in place. Loosen the retainers for the struts, but leave the struts attached until you're ready to swap the glass.
The pins up top are both are removed from the same direction (and they're held in with e-clips - which aren't
too critical - I have one on one side that's been missing for a year now, and don't even think about it any longer as something I have to remember to replace (until I wrote this, just now...lol.) But it'd be good to have the retaining clips in on both sides, especially since you're not going to be the main user of the truck. I also don't lift 'just' my glass, either - only the full liftgate - which helps keep mine in place.
On mine, one pin came out pretty easily; the other took some effort. Once the pins are out, you need your helper to swap the glass out - that's when you detach the strut mounts from the glass, hold the window upward, and slide the glass off the top mounts.
The pins will go back in easier than they came out, if you encounter resistance removing them.
Oh, and to answer your question about LWB & SWB spoilers just now - they are NOT interchangeable - not the spoilers, nor the hardware. I forget about the brake lamp (CHMSL), but I think those are wheelbase-specific, too.