I just had 2 12" mtx subs installed and they sound great but my back right speaker shakes when the subs hit a low frequency really loud. what shoul i do??

Which Speakers, The Subs Or The Rear Deck Spearkers? If They Are The Rear Deck Speakers They Are Probably Loose, Or Blown, Go Back There And Check It Out.

What you need to get are speaker baffles to fit the speakers. Also make sure all the bolts on the mounting bracket are tight. If you want to know it the speaker is blown, turn your subs off and play music at a moderate level, if its blown it will sound distorted. The highs will be scratchy and the lows will be rattly.
Baffles:
http://www.crutchfield.com/S-JbJRzhLYnFS/cgi-bin/ProdGroup.asp?g=762&avf=N

Its the rear deck speaker. It used to rattle when I played my music loud then it was fine for a while until i put the new subs in. after i get enough money should i look to put in all new speakers causwe thats what i was thinking but the stock ones sound fine except for the occadional rattling

you should try tightning them before you bin them,.,.,.,.,.