First off, that guy is an idiot. I wouldn't go near him on a bet.
Second, the only common
problem ZJ driveshafts have is grease monkeys that think they know more than the engineers that designed your Jeep.
That being said, when the drive shaft is removed, it has to be replaced with exactly the same orientation as before, or you will get vibration. Has the vibration you had before at 70 mph gone away?
If so, most likely all you have to do is get your driveline balanced.
It sounds like you have the type 3 driveshaft (CV joints). There is a procedure for installing a CV drive shaft. So make sure whoever installs it next time knows what they are doing, or you are just going to continue to suffer vibration.