I am a new Jimmy owner (2000 SLT) and I can't fined a aftermarket left rear bumper extension, mine has a simulated brushed stainless moulding about 2" wide on the top that laps over the rear quarter panel. I am vary excited about getting my truck on the road and cant believe that I cant fined this part. This truck was given to me and I know vary little abut GMC product, any other advice as to what to look out for would be grate. This Jimmy sat for three years and I have had to replace many leaking lines ie trany cooler,power steering and now I have a leek in the full system over the tank. The engine runs strong.

 

I had the same issue when I got my 96 Jimmy. As far as I know, you can't buy a reproduced bumper end for the Jimmy's. I started checking out junkyards and craigslist and eventually I got a hold of one.
Good luck!