I see they make a spool that will lock up the rear end so both wheels are powered. Do I want to do that or keep using f/r diff gears?

It depends on what kind of driving you want to do. The spool is great for a rock crawler or hill climber but if you plan on doing any turning at any type of speed you wont like the spool. I tried one in my maxx and took it out after a week,it will just spin out when you turn under power.

and it depends what surfaces your driving on too obviously

if you are a basher or a racer, and have a savage i would suggest the 4 spider diff mod in the front and rear diffs. I use 5000 wt diff oil in front and 3000 in back, truck never handled as good as it does since this mod. Not sure if any other mT's have spider diffs already or can be modified. But it works great on the savage and it only costs like 25 bucks for everything the oil sis the most at like 8 bucks a piece.

if u want a climber get it if u want to turn dont yo ceme. were can u get the diff mod and maxxover. if u did turn with power would u spin or would u 4 wheel drift?

You could get a 4 wheel drift but not really easy on certian surfaces,but on a tight track such as my local track it would just spinout. If you plan on doing any racing then deff dont get it,if you are just bashing around and kinda havin fun then...its really a decision that you will have to make yourself......if you are hill climb or rock crawl then deff.get it. The good thing is that its cheap and you can always take it out if you dont like it.

I run spools in both diffs, mainly for crawling/climbing/bashing. If you're track racing leave the spool on the shelf or at worst put one in the rear diff only.