Have you checked your brake pads? What does the disk look like. Any damage to it? Check both sides. Check your fluid level and brake lines. Does anything have fluid on it?

Possibilities include a loose rear caliper bracket, worn pads or sticky pistons (piston edges covered with contaminants, seals worn, or caliper pistons not traveling straight in their paths). The two other possibilities that comes to mind are:
(A) a rear axle bolt that isn't torqued to spec, permitting the rear wheel to slide a little in it's mounting when the rear brake is mashed; OR
(B) a really slack chain slapping against the swingarm hard when the brakes are applied.

Personally, I fully believe a rear caliper rebuild is called for -- in large part, for your personal safety. To do this right, you'll want a set of stock OEM Suzuki Brake pads (about $22 - $35), the replacement seal kit for the rear brake calipers ($12 mail order, $22 at the local dealers), a liter of DOT 4 brake fluid (Castrol GT LMA is preferred, Valvoline PowerSynth is the next option), a toothbrush, some scotchbrite, a big steel or glass (not plastic!) pan to rebuild the caliper in, and a can of spray brake cleaner. Plus a repair manual of some sort to walk you through it (it's not brain surgery, but it's too big a safety issue to just wing it if you're unsure of what to do). If you don't have a compressed air source, a cheap bicycle pump ($12, Walmart) and a rubber vacuum fitting ($2, auto parts store) will work to force the calipers out for the rebuild. Big C-clamp to seat them again afterwards.

SpeedBleeders ($15 a pair) will make the job infinitely easier and is a good upgrade, OR a brake bleeding pump ($25) will speed the post-rebuild bleeding process.

TIPS:
WEAR EYE PROTECTION. BRAKE FLUID IS CAUSTIC AND CAN BLIND YOU.
Also, wrap the bottom 1/3rd of the rear wheel & tire with aluminum foil, to keep any brake fluid from getting on the rubber of the tire or the finish of the wheel (cheap prevention).

Cheers
=-= The CyberPoet

dang poet, I want to do it my self so I can get the know-how,it cant be that hard just sounds a little time consuming. I'm lookin for a repair manual.or I can always take it in, still new only 2200 on it 4 months old.

Do it yourself. On a difficulty rating of one to five, this rates as a 1 or one-and-a-half. Yes, it can be time consuming, especially for first timers, but there's nothing in there that is brain surgery. Get a repair manual, preorder/get the stuff I listed, make sure you have the tools you need, and give yourself a full afternoon to do it without too much pressure. Take a few breaks in the middle, reread the manual, have a snack, go back at it.

Cheers
=-= The CyberPoet