If the rear is sagging, it isn't because of the shocks...the shocks dampen movement, not support the vehicle. The springs hold the vehicle off the frame/axle. You may have weak springs, they aren't that expensive to replace.

You should be able to test your rear shocks by trying to bounce the back of the vehicle...if it keeps on bouncing, you know that the shocks are completely gone...other than that, they do lose their ability to dampen over the miles/years.

Your concern about the shock springing off isn't really relative for the rears. The previous post mentioned spring compressors...that is required for front strut replacement, but the rears I believe are not under that type of tension. You may want to verify with chiltons/haynes manual, but I am nearly positive that they are not under load when vehicle is up or the shock is fully extended.

I'd stick with boge if you don't want a harsher ride.