There is rear stabilizer bar (borrowed from the VLR "V") package available in component form. This is an easy install if you have an hour to spare. The package runs about $110 + shipping. Two part numbers are identified for the Stabilizer bar ("Shaft" in gm parlance), each with different diameters. If any owners can measure and post the diameters here, it would be much appreciated. (Mine is on order, otherwise I'd post one of them.) All parts are available from gmpartsdirect.com.

Here is the info:

NAME PART NUMBER QTY NEEDED PRICE (ea)
Shaft (smaller) 15806422 1 50.01
Shaft (larger) 10339124 1 50.01

Link 15907003 2 19.59
Insulator 10317560 2 5.20
Bracket / Clamp 15142776 2 5.20
Bolt 11515775 2 2.52
Nut 11516078 2 2.52

Rear Stabilizer Bar Installation:

Two bolts needed. The control arm has a black cover that hides the top clamp threaded holes, and is easy to open up. Use the bottom bolt that holds the lower control arm for the nuts. Locations are shown in the Field Service Manual. There is adequate thread left on the lower control arm to install the bushing bracket with the nut.

According to the Service Manual for the XLR-V, the rear stabilizer shaft link nuts are to be tightened to 76 N-m (56 lb. ft.). The stabilizer shaft insulator clamp bolts are 65 N-m (49 lb. ft.) and the shaft insulator clamp nuts to 95 N-m (70 lb. ft.). I'll post pics when installed.

CCC

We recently did one of these to a customer's vehicle. It makes a nice upgrade for not a lot of money and only takes about 30 minutes to install. I have no idea why the factory chose not to put a rear sway bar on the XLR, but it can certainly benefit from having one!

Update to my previous post:

My bad. The quantity for the nuts (P/N 11516078) is 4 not 2. I hope this didn't cause anyone any heartache.
Cheers,
CC

Actually you will need 6 nuts. two for each end link and one for each of the brackets. Only two bolts are required, one of each, for the brackets.

I stand by my earlier post. On the 2004, 4 nuts are all that's needed -2 for each of the links. The upper bracket gets a bolt on each side in the upper hole, and the 1" of thread left on the bottom stud is more than enough to install the clamp, using the nut that's already there. Unless you don't want to re-use the existing hardware, 4 nuts will do the job.
CCC

I went to the web site and entered all the part numbers . I got a cost of $ 232.10. Is that right? I entered 6 nuts & bolts to be safe.