When to replace u-joints?

I have a 96 S-10 Blazer 4x4, a shade tree mechanic and need to know if the "clunk" noise made when shifting into reverse from being parked means it's time to replace my u-joints. I've check the clamps for tightness; tried shaking the shaft (won't budge); but not sure if should replace them. Any suggestions?

I went through this last year. I have a 95 ext cab w/4.3. I had it repaired because it developed a bad vibration when accelerating. Turns out that the ext cab s10 has a "special" drive shaft that contains no less than 5 ujoints and 2-3 cv type joints. The garage that quoted me $75 to replace my ujoints called me back and wanted $800. I picked the truck up, removed the shaft myself and had it repaired by the exact same place the garage would have sent it. They only charged me about $350. So basically the garage wanted $450 to remove/transport/install my driveshaft. If you have this type of shaft, you don't want to try and repair it yourself. I don't think mine clunked any more or less after the rebuild, so I can't say for sure if yours is bad or if you have that type of shaft. Good luck.

If you tried shaking the shaft in park with both rear wheels on the ground, you may not be able to see or feel any play because the weight of the vehicle is against you. Chock the wheels and put it in neutral and then try it.

A small clunk when going into reverse or drive is normal.

It is never a silent gear change.

I've been thinking about putting the rear end up on jacks and shifting gears to hear for that annoying sqeaking sound and sometimes a brief rattle when blazer is accelaerating. Squeak sounds like it's in time with the wheels or shafts turning, it goes away after gaining speed or gear shifts to D3?????

Check the cat for the rattle.

Get under the truck and slap the cat firmly and squarely with the palm of your hand and listen for loose rattling inside of it. If you hear nothing, the cat is not the cause.