I was told the cover was there as an exhaust heat shield to save the u-joint from early failure. Not sure how much truth there is to that, but my cover was off when I bought my truck and my U-joints were loose.
I more-or-less had to have them replaced at the shop that was doing my safety inspection as I was running out of time for getting my truck legally imported into Canada. The shop that did it charged me somewhere around $1,000 to do it. I was not impressed. After that I put the cover back on.