I'd think that a quick test of the front drive axle would determine if the u-joints would need to be changed.

Get under the truck and grab the front drive shaft when then truck is off and the 4wd is NOT engaged. Then try to rotate the shaft. If there is play that then stops, it is then likely that the bearings in the caps of that center the ends of the cross are worn out and have dropped out. You will want to look for the driveshaft yoke being able to rotate a bit before it causes the yoke on the other side of the u-joint to also rotate.

Here is the first video I found that discusses how to look for a bad u-joint. This guy also adds another good test to see if the yokes move laterally. (important part from 1:36 to 3:59). FYI: His vehicle is a Jeep with a solid axle and u-joints at the front tires.

 

May also check the front wheel bearings.

 

The video that christine_208 posted is actually the same video I watched originally that got me started on this quest. Around 4:25 in the video he say that a lot of times when those front u-joints go bad the driver will hear a clicking at slow speed going straight down the road, but christine_208 said his vehicle is a Jeep with a solid axle and u-joints at the front tires. I think I've been hearing that my girlfriend's 1995 GMC Jimmy is different. Basically no u-joints at the front tires like the Jeep in the video has. If I'm correct does anyone have any idea what the clicking noise is. From my experience CV joints usually only click when turning and these don't.

 

If the 95 Blazer has 4WD, then it most certainly and without a doubt has 4 CV joints on the 2 axle shafts going out to the front wheels from the front differential assembly. There is absolutely no room for argument about it. The CV joints are best replaced by replacing the entire axle shaft that includes both CV joints with the shaft. It requires removal of the front spindle for access, so not an easy repair for a novice and they are relatively expensive unless you get them from Rock Auto.

It also has 2 joints on the front driveshaft between the transfer case and the front differential. The front one is a u-joint that is serviceable using a press, and the other is replaced with the entire driveshaft (or by a driveshaft shop).

What may be likely is that the mechanism that locks in the right front axle when 4WD is selected, is partially activated/partially engaged and clicking while in 2WD. So I think shot gunning U-joints and CV joints is really a poor way to proceed.

Here is an article on the clicking of the locking mechanism. At least check for the sound to disappear when 4WD is engaged - and also check for 4WD engagement (if you jack up the RF wheel and leave the rest on the ground - then with the engine running and 4WD selected you should not be able to turn the RF wheel by hand - select 2WD and then it should spin by hand). https://blazerforum.com/forum/tech-a...4hi-4lo-41226/

If the front axle engagement is the issue, then disconnecting and plugging the hose on the servo under the battery tray will most likely make the clicking go away at zero cost. Repairs can be made later, but in the meantime 4WD will no longer work.

Good luck and best wishes for a successful repair!

 

I was able to replace the front driveshaft CV joint on my '02; I think the '95 would be the same..??
This CV joint would "click" like crazy when going slowly in reverse.
Was not too difficult to change-out; had to do it twice as the first CV joint fell apart in a few miles; defective part. RockAuto sent me another one which has been fine for several years now.
Getting the front driveshaft out is not difficult. Getting it back in is. Easier to pull the driveshaft end out of the transfer case than it is to push it back in.

As stated above, if either front axle CV joint boot is torn or there is grease that has been thrown-out of either one, time to replace the entire assembly. Getting the half-shafts out is not easy...lots of work required.