Might be that he has seen a lot of rear shafts fail. Could also be that he is just trying to sell some more product.
This might not help much, but......
I wouldn't be as concerned with the price as much as I would the shops experience. Sometimes it's hard to know what kind of reputation they have, but it is definitely worth it (imo) to research anyone that does major work on your Jeep. I have no idea how involved this installation is with this particular lift (cutting, welding, relocating, etc).......but there are VERY few people that I personally trust with lift installation.
Do they have a good reputation? Customer references? A website featuring their work? How many RK lifts have they done? How many Jeeps have they lifted? How many of those Jeeps were JK's? How long have they been in business? What is their specialty?
Even if that is a straight-fwd lift to install, there are simply corners that should not be cut- brake bleeding- relocating, correctly torquing bolts, cycling the suspension, looking for rubbing/ wear issues with all the suspension/ driveline components, etc, etc.
If they are a trustworthy shop- then their price may be reasonable. If they're using your Jeep for a guinea pig, then look elsewhere. Don't pay anyone to be under your rig saying "Hey- lets see what this does." or "I wonder what this thing is for?"
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