I was originally going to say "you've got the scope already, just upgrade the rangefinder", but if your scope is not returning to zero reliably then it has a problem. I haven't heard this about Zeiss scopes before, and my Zeiss scope seems to hold zero just fine (but, it's not the target turret style and I don't crank the adjustments alot).
I guess, if it was my decision to make; I'd send the scope in (if you are certain it's the scope and not the load/gun/shooter/conditions/shooting position), and buy another scope to have for backup or use in the meantime.
Nearly everyone I've talked to has not been very impressed with the Leupold rangefinders ranging ablilty. So upgrading to a Leica is probably important at some point if you want to shoot much beyond 500 yds.
However, the scope will cause you way more grief than the rangefinder until that issue is resolved or replaced IMO. Depending on the size of your target; If it's returning to within a 1/4 or maybe even 1/2 minute, that's not really a big issue until you get out there a ways, but if it's returning to within only 1 minute, then that seems like a big problem that should be fixed asap.