I have 2013 Grand Cherokee Altitude and needed a roof rack. I was going to go with Tuned8e's solution but I did some more research and came up with another setup after considering the price of buying the chrome rails and a roof rack.
I went down to a local roof rack shop and they let me look at all of the Yakima Landing Pads. I found one that I thought would work on my current plastic non-functional roof rails.
I bought two 2-packs of the Yakima Landing Pad #11
for 78 + tax. Landing Pad #11 is designed to attach to a roof rail with just one bolt per landing pad.
I had to use my Dremel to cut out the fronts and backs of the landing pad to fit on top of the black plastic rail (the cut line is shown in the image in bright green).
I found the Yakima Control Towers with keys on Craigslist for $100 instead of paying full price of $198 + key cost + tax. I also bought the Yakima 48" crossbars on Craigslist for $50 instead of paying full price of $89 + tax.
To install, all I had to do was remove the second bolt from the front and the second bolt from the back from each of my plastic rails. Go to Home Depot and buy four 50mm M6 bolts, some washers, and some silicone (to be safe) for about $10. I used 5 washers on each landing pad to get the proper length for the bolt. Then I applied a little silicone to the bolt before I screwed it down to a very secure fit. The rubber "feet" that come with the landing pads really help ensure a secure fit.
Total cost was about $250. Took me about two hours to get the right cut on each of the Landing Pads and install. I was able to keep from adding any more chrome (which I hate) to my Jeep. And I'm very happy with the final look and feel of the rack. It is very solid.