Are you sure the axle is bent? I ask because you said you put new OEM tires on it which in some cases are notorious for developing flat spots on them when the bike sits for a while. When riding with flat spots it'll definitley feel like the axle is bent but the ride will smooth out after ridng a while.
If the axle is actually bent, I'd go back to the shop asap and talk to the manager or owner. Good luck.
Current Ride (Purchased new in Oct. 84) - 1985 Honda ATC 250R w/ ESR 310cc big bore kit
Wife's Ride - 2009 Polaris RZR 800
Kid's Rides - 2009 Yamaha Raptor 700, 2008 Yamaha Raptor 250
Former Rides - 1975 Honda XR 75, 1979 Yamaha IT 175, 1978 Honda ATC 90, 1980 Honda ATC 110
got sand?