So I went to pick up the Jeep from a local shop that swapped out the OEM 3.73 gears to Dana 4.88's with Motive master install kits. He has done several gear swaps in Jeeps for me over the years and i've always had excellent results. I pulled out of his drive and the improved acceleration was awesome....no gear whine at all....and then as the speed approached 50 mph, a harmonic vibration started that was enough to get the coins vibrating in the coin holder and the dash to rattle. The Jeep had zero vibration before the gear swap. I left it with him and he's going to go back through it tomorrow. I did a search and I see this is an issue that numerous people have experienced after gear swaps and several seem to have not gotten it resolved. The Jeep is my DD so I need to get it figured out. It has 4" EVO coils with Rancho shocks, AEV Geometry brackets, 34" Falken AT3 tires on stock wheels, JKS 1.5" spacers and adjustable front and rear track bars. The drive shafts and all control arms are stock. What concerns me is there was zero vibration (up to 80 mph) before the gear swap. I pray that he finds something obvious tomorrow and it's vibration free, but with my luck he won't find the cause. If that's the case......before I go back to the 3.73 gears....what should I check or do to try to identify the cause of the vibration? I would hate to buy new driveshafts and control arms and that not fix the issue....i've read threads where people have done that and still had the same issue. Any suggestions appreciated!! THANKS!

 

Outside of wheel balance, which, you never know...
Otherwise, you already identified where I would look first... driveshafts, and you already identified what's required when you go to double cardan shafts, control arms.
With driveshafts, pinion angle becomes critical (well, driveshaft angle, so, pinion is what gets you there...), so, figure on adjustable upper control arms to get that dialed in... lowers would be nice too to get the wheels centered in the openings, but, not a necessity.
I've ran all sorts of gear combo's in various Jeeps/Mustangs and have always gotten the harmonics associated sorted... Mustang took 2 driveshafts.. first one was an economical attempt, next was spendy, but got there.
Good luck.

 

OK...so the pinion flange on the rear axle had excessive run out. A new flange went on today and it's smooth and quiet with no vibrations whatsoever. I'm unsure if the flange may have been damaged during the removal/reinstall or it may have always had too much run out and the addition of the 4.88's which causes a faster spinning driveshaft may have highlighted the issue with vibration. Nonetheless, i'm grateful that it's resolved.