car help wanted (thumping jeep)

[Artemas2]

Any car mechanics around? My local shop's diagnostic skills are leaving a bit to be desired and my check book is about run out on this thing.

I have a 2001 Jeep Cherokee that thumps like a bass drum on left turns and after bumps.
The issue started after getting new front brakes and rotors earlier last month, I mentioned that to the shop, but I don't think they took me seriously.
The shop claims they can't replicate the issues and when I showed them a video they only asked me if the sound was turned on because they could not hear anything At their suggestion I had all the front end bushings, sway bar, control arms and some steering parts replaced with no change.

[LOKNLOD]

Not sure what's going on but let's ask some diagnostic questions:
Only on left turns - yes/no?
Continues during straight travel after a left turn - yes/no?
Does a right turn while it's thumping cause it to stop -- yes/no?
Seems to be speed dependent, rate increases as you go faster - yes/no?
Does braking change the thumping - yes/no?
Have you tried to observe it with windows down, from passenger seat with someone else driving, etc - yes/no?
Any modifications to the jeep? larger tires or the like?

[0ddl0t]

How rusty is the undercarriage?

[Artemas2]

Not sure what's going on but let's ask some diagnostic questions:
Only on left turns - yes/no?
Continues during straight travel after a left turn - yes/no?
Does a right turn while it's thumping cause it to stop -- yes/no?
Seems to be speed dependent, rate increases as you go faster - yes/no?
Does braking change the thumping - yes/no? (will need further testing)
Have you tried to observe it with windows down, from passenger seat with someone else driving, etc - yes/no? had someone else in the passanger seat, they also followed me around outside, but could not hear anything outside of the car
Any modifications to the jeep? larger tires or the like?
it does have a helper spring in back, and the engine is a replacement neither of those are new additions. everything else to my knowledge is factory. Tires are a general purpose all season factory size tire

Actually to clarify some of those it does stop tumping after the wheel is straightened out, but it takes 50 yards or so, I don't think braking is affecting it, but I am not 100% sure on that

How rusty is the undercarriage?

It is rusty on the underside. over 260,000 on PA winter treated roads, it was undercoated about 8 years ago so its still mostly surface rust.