I have a 1988 fc convertible, they came stock with an open diff. Is it possible to take the gxl or gsl models diff and bolt it up to my rear end?

 

yes. the 86-88 GXL, and 89-90 GTUs had a limited slip. the GXL automatic had a 3.9 same as your vert, the manual had a 4.1 which would pep the thing up a bit, and the GTUs had a 4.3 which would make a big difference

 

So I should be able to take a diff from one of those two and it’ll still fit up onto the mounts and axles, etc.

 

yes