I just purchased an '06 low mile 19k LR3 and the cert costs $2k extra. The dealer may not want to do for a number of reasons. First, tires, if the the tires are worn (and they wear quick on a 6k pound vehicle) they have to be replaced and LR requires them to use the dealer tires for replacement, costs them close to $1k just for tires, I negotiated a credit and bought my own tires (didn't want the crap goodyears). Plus if anything else is wrong, brakes etc, it may cost more to cert than they can charge. Also, if truck had accident repairs they may not be able to certify. Have them pull repair records from their computer, it will give you an idea as to what work was needed on the vehicle. (ie: my just had oil changes and a few recall updates) Also make sure to see carfax report.
On a low mileage '08, it may not make sense for you to cert b/c it only gives you 6 years from the date the truck originally went into service and 75k miles total. The balance of factory warranty would cover a large chunk of that time/mileage.
If you do go forward & have it cert, make sure you specifically tell them to do all software updates. It seems LR doesn't automatically apply these to a truck unless the customer has a problem and/or requests it. Good Luck!