how to unstick dried pineapple chunks that are stuck together

If you want to separate while retaining shape of individual pieces, freeze then pry.

May cause small change in texture upon thawing

Depends on what you want to use them for. If you are using all of them in a cooked preparation, easiest way is just to rehydrate them- in water, in rum, whatever works best for your recipe. If you only want to use a few of them, there are 2 muscle-intensive options that come to mind: 1) bash them with a hammer (best if you freeze them first) 2) Cut off a chunk with a chef's knife/cleaver.

Mostly though it just depends on what you want to do them.

You may be able to pry them apart with the flat of a knife - like a butter knife - if there is enough of a gap anywhere to get the knife-tip in. I would suggest a twisting motion, as it's more controllable than pulling or leveraging apart and safer than just pressing downward with force. This doesn't work so well if they're small enough or stuck together enough to have a relatively smooth surface, but with pineapple chunks it seems a bit less likely.

If you don't have a gap to start with, you should be able to make one by pressing the knife in with force - metal knife tends to trump dried fruit - but you will want to be careful about angle and force to make sure if the knife jumps or the pineapple gives unexpectedly you don't end up with that force landing somewhere unpleasant (like a hand. Ouch.)

If you don't think you can pry safely, you might take a sharp knife and cut into the pineapple clump. This may help if it's somewhere hard to maneuver (like a jar), so you can take smaller clumps out and bash or pry without worrying about the jar, or else if the pineapple is really really stuck, due to storage issues. If it's stuck hard and smooth, you might just carve it up into cubes and use it that way - not likely to crumble on its own if effort won't pry it loose.