I too have been experiencing the same in second and third. Not a chatter, but a vibration. It could be your rear end (ring and pinion). Let me know what you find out.
95 Yj I-6 Newly Rebuilt
Dana 35 Rear Newly Rebuilt
4inch Skyjacker Suspension Lift
2inch Shackle Lift
Skyjacker Nitro 8000 Shocks
Flowmaster 50 Series Delta Flow
33 X 12.50 X 15 Goodyear Wrangler MTR's