Yes you can. 46u would be the one to ask though as he's got an MT1, but upgraded to MT2 18ss specs.
I found I only needed to put the SS diffs in my MT2 after I installed an upgraded engine...only 1 tank on the dirt and the stock ones were destroyed!

However, if you are still running the RTR w/T-15 or 15FE, I would recommend perhaps waiting until the stock ones go before upgrading. However if you put in a larger powerplant, then yes, you'll definately need the 18SS diffs!
--RCNZ