I guess nobody has properly welcomed you. You may find that it is best to spend some time using the "search" function when you have issues with your truck. You would be amazed on how much info is out there from other owners.Have these been prevalent problems/any thoughts on Nissan's quality or lack of?
Now, since your rear axle was leaking I am going to assume you have the m226 (Any Nismo or 6m). You may want to search for the 'axle vent mod' so that you don't have that issue again.
You mentioned that both your front and rear U-joints are bad. This once again is a common thing. You may want to search for "u-joint replacement' and see what comes up. I quick overview is that Nissan uses sealed units that just go bad and can not be re-greased over time.
Now if you have any other issues, take an hour or so and read and read and read. The 2nd gen Frontiers have been around since 05 and a lot of members have put them thru very different situations and most problems have been seen by now.