I have used heat guns and other products on faded plastic - IMHO every option looks like **** after a little time. The heat gun is also a short term fix(from my personal experience - It looks really good for a short time). The bad thing about heating them is after time they can begin to crack on the surface. Yes it does bring back most of the new color but does not make them look new for very long. If you go the heat gun or torch route then you want to make sure they are as clean as can be before starting(also don't do in a dusty environment because you will get a lot of imperfections and/or debris, bugs, etc. in the heated/melted plastic top layer).
I like subatomicgenius's idea of looking for some new replacements. There are tons of members removing the plastic parts on their JK's for metal parts and you could probably score a pretty decent deal on some new(er) ones.
Before buying new though, I would try the penetrol. I have heard good reviews about it and it may be worth a shot before getting some newer replacements. The heat gun and torch is not a bad option, but it is just a temporary fix like all the others from my personal experience. Heck, none of the suggestions are bad. Pick one and try it. Worst case scenario you get your hands on some new ones and all is good!