I can confirm that the Spec-R diff with inserts but no rings "works" on an Mi4LP. Double check to make sure the blades aren't bottoming out. I can't vouch for long-term functionality, as I only ran a few race events with a car like that before I upgraded it to CX axles. I also can't vouch for the 3/3.5, since I've never run those cars.
As for the front... The gear diff will work in the front of the LP. The pulley is more offset than the spool, though, so the belt gets a bit of angle to it. It does seem to clear the top-deck OK, though (barely). The gear diff actually has even more pulley offset than the stock ball diff, too, so you get a bit of angle in the belt to the rear as well.
I did notice that for some reason when I set the front gear diff to the low position, it was binding on something really badly. I didn't figure out why, though, and just put it back to high, since that's where I normally run my diffs.
EDIT: You can also get this gear diff
http://www.spec-r.com/default/index....roducts_id=209 if you don't need the slider inserts and rings. It'll save a few bucks over the Schumacher part #.
-Mike