Unless you did the service yourself I would not trust that to have done anything more than change the oil. I have bought cars which have a just had a service, book stamped etc, and the oil filter hasn't been changed! (they are usually to go racing in and then scrap so it doesn't matter to me)
The service interval on a defender is quite short to begin with (6000 miles for early ones, longer for later models). But they require more maintenance than a normal car. They are not the sort of vehicle you can get a garage to do a basic service on and then not touch it again for 12 months until it has the next one. I would say I average some sort of work on mine every couple of weeks, but I proactively check things and investigate any changes before they get worse.
If you are worried about it, it might be worth giving it a bit of an overhaul, and a very serious service. Wheel bearing kits are relatively cheap and only take about half an hour and basic tools to replace yourself so you could do all four of them and rest safe in the knowledge that they are in good condition and well greased. But it may also be worth using the opportunity to give the rest of the vehicle a thorough check over as well, brakes, drive train, full fluid service etc and if you find anything else get it sorted. This would hopefully give you a bit more confidence back in the vehicle as you know it has all been checked, replaced and maintained to a good standard.