Can i drop in a 92 TC from an XJ into a 90 XJ? I mean does it just drop right in?

 

as long as its the same tcase you can some are np231 and others are np242 also they have varying spline counts

 

They are both NP231s. How can I tell if the splines match without taking mine out first?

 

Yours being a 90 it could be 21 or 23 spline input. The 92 is definately a 23 spline.

If the 90 turns out to be a 21 spline you could swap input gears, but that involves tearing into the transfer case to do so. Not difficult if your a decent wrench.