I just bought a 93 xtra cab in great shape, the front suspension is stock but the rear has blocks under the leaf springs, im wondering if they did this because of rear sag are they safe etc.

 

Most will say they are not recommended. The greater the distance between two points the greater the chance it can move. Little thing called leverage. That means on hard cornering or while 4-wheelin the block can fold, twist, turn or worse yet break or crumble. This is mainly true if the block is taller than it is wide. Most people never have problems with a 1 or 2" spacer block for leveling.

I would double check the nuts on the u-bolts to make sure they are tight.

 

2" should be ok but anything more than that you will have axle wrap and like what james dean said here leverage is bad on elongated u bolts

 

Any amount of block will increase the leverage on the spring and increase the chances of axle wrap/wheel hop, etc and generally lead to shorter life for the springs.

So long as the u-bolts are tight, though, there isn't anything inherently unsafe about blocks.

 

dont get aluminum ones (el cheap-o from auto zone) if you do much wheeling they could buckel. my buddy's 93 truck bent his blocks and tore up all kinds of stuff...drive shaft, outputshaft, etc. the blocks had so much torque on them that they caved.

 

I'd say it's safe in moderation.