I've seen Rugged Ridge diff skids for about $70 and their heavy duty diff covers for about $130, which is about what Poison Spyder charges for their bombshell covers, and a bit more than what Riddler charges for their heavy duty covers. You may have to paint the PSC or Riddler covers. If you are never going to take you jeep offroad - a true Mall Crawler - then I can think of no need to spend the money on any underbody protection. If you are going to buy the diff skid, I would really consider going to a heavy duty cover instead of a skid. I just put on an old-style D30 PSC bombshell cover up front and a Riddler D44 cover in back. The D30 PSC cover weighs about the same as the OEM D30 + D44 covers combined, stock covers are very thin. I banged one up on a green trail at Rausch Creek, stock cover leaked after that, I was probably lucky not to have peeled the cover off. I'd probably have to bust an axle before I'd damage these heavy diff covers. Last time I checked, PSC had a disclaimer on their site about not using their D44 bombshell cover with a JK since there could be contact between the diff cover and the track bar.
Looks like you need to take out the bolts to install the diff skid, don't know if the old gasket seal will need to be replaced, or if you can just replace the bolts and re-torque with the skid in place. I'm assuming new hardware is sent - longer bolts - with the skid.
Underbody protection is not cheap, but the next thing to consider would be an oil pan skid (I have a combined oil pan + transmission skid, as well as control arm skids - to me these are a much lower priority for offroading than the oil pan skid). Next piece of underbody pritection I would consider for offroading would be the AEV D44 rear diff slider.