NEED HELP: problem w/ my rear diffrential
 

hi there! i have a problem w/ my car (nitro fusion).
i've dissassembled it to clean its parts and opened the rear diff. coz some dirt stuck in the pulley. i dissassembled the diff and cleaned the inside. then assembled it again and forgot that i don't have any more diff lube/grease. as i read the manual, it says "use schumacher diff lube ONLY!"
i didn't took notice for that. then i went to run my car and after a while, i've notice that the front tires are the only one engaging. i've help up the rear tires and they are not engaging.
i took a look at the diff to see what's the problem. i saw the diff setting screw is loose. so i tighten it. then run my car, again after a while the car runs only front wheel.
do you think the problem is the diff lube/grease?
i've put too little hpi silicone on that diffrential.
i have no idea. i'm looking on buying an assembled diffrential for that. but im thinking twice coz it's $60. it might not be the solution
need help pls!