Hello and welcome to jeepz.com .
although I don't own a lifted jeep and am not experiencing the issue currently , I will say you've done all you could to remedy the problem as reasonable and feasible as possible.
youve inspected or had inspected the steering and suspension parts and replaced what was worn . You've had the tires balanced as well . That is a big concern in lifted jeeps.
Larger than stock is the biggest concern suspension and tire size equally. Although your tire size is t over stock , the lift is a tire size versus lift ratio issue. That is so,etching only a proper four wheel drive specialist can advise you on what is needed.
Realizing that you've joined jeepz.com for help as there is always the possibility a specialist will want $$ and all your trying to do is find out why and has anyone else had this issue and what they did to successfully fix it.
With my limited experience of lifted jeeps , I not only bring attention this this thread for assistance but will say that from your posted description of your jeep in its present state , that will give more info of what to work with. That is very helpful .
BTW , is the 4" lift all on the suspension ? None on the body ? No transfer case drop ?
I ask because the drive line is altered in terms of drive shaft angle and axle angle with regards to universal joints . Although your not experiencing u-joint issue , drive shaft balance is a consideration in this equation as well. Drive line vibration does not directly contribute to death wobble but the vibration can sometimes be mistaken. I'm sure you know the difference.
the shake you speak of at 55 mph wouldn't be driveline since that would be an issue under load . The shop that did the replacement of worn parts did what they could to deal with the obvious. That is respectable . To obtain what's really going on , a specialist needs to be consulted.
Yes , this will be a fee but trusting all the $$ spent so far was needed and will contribute to finding the problem versus the fix quicker and more feasible is your concern.
You will be asked if the trouble is felt on straight line , curve , cruise or acceleration.
The on,y death wobble I've ever experienced was on curves. That was a track bar.
Yiuve replaced yours so it more of a body verses suspension versus tire related issue. This is why it's not easy to find on a jeep much less on a non-stock jeep. Aftermarket gets blamed of kit selection and other reasons . You'll have to find a shop that deals with this that can of course be trusted that IS a specialist on lifted jeeps and four wheel drives. Firestone will not be too helpful since they deal in stock configurations. All that they can do is check four wheel laser alignment. That isn't enough for a lifted jeep . The suspension flex goes beyond stock travel and therefore exceeds stock specs. This is why the problems surfaces at the specific speed it does in addition to road character. Only the specialist can find it knowing the condition of the jeeps chassis versus all the other posted items with the knowledge and experience they have.
Realizing your jeep only has a modest lift and the problem shouldn't be so hard to find , it still isn't found . I do suggest that chassis alignment be checked to eliminate that as a potential cause before too many charges for different services start adding up $$.
When it comes to jeeps , anything is possible . Including chassis out of spec. Hopefully not the issue but since I'm currently experiencing a left side tire that wears unevenly even with new ball joints and correct camber spec , I probably have a tweeked axle and so I know it can happen and the owner not aware. I suggest interviewing a specialist. Enough money was spent. Hope this helps.