Then don't waste your money. If you must spend, do so wisely, get more bang for your buck.
Your SPID sticker says GU6.
Unfortunately, GU6 is 3.42, which sucks sweaty arse.
You could very easily upgrade that RWD axle from 3.42, to 4.10, AND add a torque sensing limited slip differential, for UNDER $1900.
(That $3800 engine - before installation - could be replicated or even improved upon, for $2850 installed, by a clever shopper with time on his side.)
Back to topic:
4.10 would actually be a good investment, as your metro / urban stop'n'go MpG would actually improve.
4.10 MIGHT cost 2 - 4 highway MpG, depending on how you drive it, but it might be worth the extra SMILES per gallon to you.
My Tahoe has GT4 - that's 3.73 - and I'm sorely dissatisfied; I want 4.10 myself, as soon as I can afford it.
If I had your XL, just before the pcm retune that you should also do, I'd've corrected GM's mistake of 3.42, and upgraded to 4.10.