What things are you trying to diff? Two buffers, backup history, ... what?
I'd like to replace all diff functionality within slickedit.
But if not possible at least replace diffzilla for Backup History comparison would be fine.
I don't have the time or desire to try this myself, but, for backup history, in the module tbdeltasave.e, you'll see there are 3 places that call a Slick C function called diff. You could replace these with a call to your own slick function. Two of the calls are passing "viewids" and one is passing a filename and viewid. For the viewids, you could probably write the specified buffer to a temporary file on disk, then call your own diff tool. Diff tools I know of usually accept the names of two files to be compared, as parameters on the command line. If you modify the file in your external diff tool, you have the problem of getting the modified file updated in SlickEdit - not sure how you'd do that - your slick function might need to ensure the file was saved to disk before calling your diff tool, then always reload it afterwards, but this will screw up slicks "backup history" deltas and defeat the purpose of this great feature.
You can also see the SlickC function diff call the built-in Diff function - (see line 18451 in builtins.e) and it probably uses vsvcs.dll to do the diff - so you could also try to replace the calls to Diff (the builtin) with your own SlickC function - that would catch more diff operations but you'd have to figure out all the options etc.
Graeme