Can I swap the rear half of a driveshaft between 2 driveshafts? Can this be done without any bad things happening? I want to just slide out the spline from one and slide it into the other. I know this might cause some balacing issues.. but can't a driveshaft place fix that?

On some drive shaft halfs there are tiny arrows imprinted at the ends of the shafts, where they connect (splined ends). One is to align these marks. That should work. If no arrows....... I would look at your shaft (both sections) and note where the balance weights are (little patches of metal) then try to rematch the marks on the swap....or take it to a shop that can balance drive shafts.

Enjoy.
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84, 242GLTi, 167K, 4+OD, in rehab; 89, 245 DL, 252K, M 47, daily driver; 93, 245, auto, 167K ; and other toys.

Yes, I've done that lots of times on a variety of cars, often in an effort to find a pair with minimum play. Obviously you have to align the yokes correctly.
There may be a balance issue. I have never detected any imbalance, but may be I have just been lucky. I'm sure it could be corrected.