Sounds kind of sketchy to me. Basically, every 4th gen's rear driveshaft needs to be replaced. They made a TSB and I think replaced them under warranty for the Lexus (same drivetrain) but us 4runner owners were left out in the cold. I think there's been some attempts to get a class action or something else to get them done without much success. If it was killing people they'd jump but just pissing off a bunch of owners of used vehicles doesn't have a lot of teeth.
I use Mobil1 synthetic grease because that's what I have loaded in my gun. About any kind of chassis lube will work. There are a handful of threads on here with people extolling the virtues of various lubricants. At the end of the day I don't think it really matters on this one. The slip yoke moves a whole inch or so in it's travel so it's not really a "high wear" item. That's why I find it sketchy they're trying to tell you it's worn.
If it hasn't been done by you or someone else lately the driveline needs to be lubricated. Both u-joints on each shaft and the slip yoke have grease fittings. Put grease in the joints until you see it pushing out some of the old grease (think playdough fun factory). For the yokes there's a lot of info. I put maybe 10 or so pumps.
If you put enough that it starts to fill the void between the two parts (you actually see the shaft parts start to press apart) then drive very carefully for the first few miles and give the grease some time to migrate around in the cavity. Maybe the dealer went ahead and serviced the driveline as part of the "clunk" service. Check your receipt or ask the service manager first.
