The springs sagging, especially in the front, is pretty standard for Sammy's with a SPOA conversion. On my SJ-410 I picked up an extra set of leaves and added an additional second number 2 (the long one) leaf to the pack, that gave it the extra strength needed to keep a neutral, or flat, pack without affecting the ride to much.
If this is primarily an off-pavement rig I would drop a spool in the back and a Lock-Rite up front. It's a simple combination that will allow you to pretty much go anywhere and the lock-rite will allow you to steer without to much of an issue. Plus if you are in 2wd the locker will act just like an open diff for steering where as a spool up front will still cause problems unless you have one of your hubs unlocked. I'm not a big fan of putting spools in vehicles that see a lot of pavement, it causes a lot more stress on the rear axles. If you plan on driving on pavement a bit I would recommend a Lock-Rite front and rear.
Remember to run your vent lines for the axles up under the hood. Otherwise you will have to replace your gear oil after every run, instead of after every other run.
For gearing what size tires are you running? Trail Tough has a sale on Transfer Case gears so you might want to check them out at
http://www.trailtough.com and look at the options available. They also have Chromo axles front and rear if you decide you want to upgrade to some stronger hardware.
