Is it normal that these trucks have a little bit of a rear end sag in the suspension.

 

is it normal? yes.
is it right?

for the zr2 the most common fix is an add-a-leaf, or a set of lift shackles*.
search both to get more than you ever wanted to know about each.

*shackles are cheaper to the tune of $60

 

ABN31B lied.... the best way is a 6in superlift.... lol not the best but another option

 

got me there