It's a bit more difficult than just swapping in the transmission, plus the 42RLE isn't exactly an upgrade. I'd take the 3 speed 32RH over my 42RLE any day.
Contact Teraflex and see what they have to say; it may be as simple as swapping the input shaft in the t-case to one that will work with your transmission.
That said, read a bit before installing it. I would buy one myself if I hadn't heard of so many people breaking theirs. You can find a few who have had good luck, but for the most part it seems like they break down a lot. I'd rather get the 241 t-case if I was looking for a 4:1 case. Of course if it's free, you can't really go wrong.