Ok, I know this has been beaten like a dead horse but I thought I would post my findings on this subject anyway!
I just took ownership of a 2011 Wrangler 70th Anna. It has less than 1200 km on the clock and has the rattle when put into 3rd gear and accelerating. So today I decided to do an experiment to actually see whether or not it was indeed the shifter knob making all the racket or was it actually coming from the gearbox.
So I removed the shift knob from the shifter handle and went for a drive. Low and behold, I shift her up into 3rd and there was no more rattle, she was quiet as a mouse, well as quiet as one of these trannies get anyway. I continue on to the next light and I am stopped so I decided to reinstall the knob to see what would happen. The light changes and I coax all 202 ponies up to 3rd gear and whalla!!! there is that annoying slightly audible rattle once again grinding into my psyche and curdling my blood like fingernails across a chalkboard!
So once again just to make sure and reassure myself that I was not just hearing things or going crazy I repeated the procedure with identical results. So it would appear that yes indeed it is not a trani internal problem but a cheep plasticky shifter knob problem.
When I get home I am going to remove the shift knob and apply some silicone plumbers tape to the handle and re-apply the knob to see if that might not tighten things up and stop the rattle.
I'll keep you posted on my fix.