71’ K5 Blazer Suspension Upgrade (where to start)
 

Changes/upgrades really depend on what your intended purpose is for the truck. If it is mostly going to see pavement with an occasional trail, then staying stock is likely going to be the best. If you're intending on rock bouncing the thing, then that changes the nature of the answer completely.

The next point of clarification is whether you are looking for help in what to replace, regardless of its age or level of wear (basic rebuild of suspension/steering components) or if you only intend on replacing those parts that are exhibiting out of tolerance performance.

As far as the stock suspension goes (all of this assuming you have a 4wd K5), you'll have a solid axle both front & back held in place by leaf springs. The front will have a sway bar (unless someone has removed it). It is possible for the springs to relax or (worse) crack over time, but if the ride height is where you want it now, you may just need to go through and replace all of the bushings in the springs & hangers to get them up to snuff. Other replacement parts include the sway bar bushings, steering tie rod ends (4 total - 2 on the 'dog bone' and 2 on the cross-over bar), ball joints on the front axle, and the shocks on all 4 corners.

If after getting all of the bushings/joints replaced you still have some slop in the steering, it is possible that the steering box is worn. Before jumping to that conclusion, I would recommend replacing the 'rag-joint' (rubber disk that mounts the steering shaft to the gear box). If that doesn't get it where you want it, a reman box might be in order.

Steering stabilizers... If you are running larger tires, then it may benefit you to run a stabilizer. I still would recommend getting everything else tightened up as much as possible before going that route though. I have a single damper on the steering in my K5 and run 33x12.5 tires. I have had some shakes (death wobble) out of it, but that was fixed by replacing the tie rod ends and front spindle bearings (which is another place to look for play).

There are some pretty simple tests that can be done to look for play in the various locations/components, but again, it really depends on what your intentions are. What I have provided above is simply a list of possibilities.