Driving -1 vs. -2

As guesses, two possibilities: first, is the ring gear safety-wired and the nine bolts tight that hold the gear to the differential? If loose, you're on borrowed time until the ZF disassembles itself for BIG BUCKS. An inspection probe inserted through a removed side plate stud can sometimes see existing safety wire, but if there's any doubt at all, pull the transaxle and visually confirm bolt tightness. The LEAST amount of damage from loose ring gear bolts is a destroyed lower cover. The most is split cases & wrecked gears.

Second, lurching etc in turns can come from the posi clutches catching, then slipping, then catching again. Can be fixed by using posi-lube additive that allows a little clutch slippage in turns. Some 90-wt lube already has the requisite amount added, and L. Butfoy says it doesn't hurt & may help this condition.

L Butfoy just rebuilt mine, so I'm assuming it's perfect, which is probably a mistake.

I suspect your lurching at low RPMs is a lean condition with your carb. Is it less when the engine is warm? If so, that supports the lean theory. I seriously doubt it is a ZF issue - especially if Butfoy was recently in there.

Did you fill the ZF with "Psi-traction" gear oil?
if not it can cause the clutches to slip in the ZF. causing a slipping surging feel as the clutches slip.