Could be either or both. Are you sure you have ball joints?(some axles have kingpins) You should first rule out the ball joints as the problem.
Raise the front of the truck and check if there is any "play" by grasping the tire @ 12 & 6 and try rocking it. This is the "quick" method of checking. There is another method that requires a strain gauge and turning the wheel, but I've never done it so I cannot advise here. If you have even 1 bad ball joint I and "many others" would recommend changing them all along with any other parts found unserviceable along the way.
Can you do it yourself______Short answer is yes, is it as easy as changing you oil____Hell no. You should invest in a ball joint service kit (not really expensive) and dedicate a whole weekend "just to be safe" There are many threads here about replacing ball joints...."search function"
As to the alignment, #1 find a shop with an "old timer" doing the alignments (he is usually more versed in SFA vehicles) #2 make sure they have the alignment shims "in stock". My experience finding a shop that has the shims is, in a word___exhausting. I have found "NO" shops other than a dealership that has the shims. #3 Determine from them if they go "within spec" or "to spec" There is a huge difference (especially with tires that are within a couple sizes larger than stock.) If you have 35" or larger tires look for a place that is experienced with 4x4 alignments, as there are different angles that need to be achieved for best performance.
Here are the spec's I have for the blazer alignment
____________________________________L-Caster_______R-Caster________Camber________Total Toe
Blazer, Jimmy Full Size 77-91_________8.00° ± 1.00°____8.00° ± 1.00°____1.50° ± 0.75°____0.12" ± 0.05"
Hope this helps, and again "search"----- it is your friend.
