Many hosting companies, like mine, won't have the AJAX Extensions installed in the GAC for quite a while, if ever.   No worries, you can just put the DLLs in your Bin folder.   Since MS hasn't graced us with the DLLs, so you'll have to get them out of your GAC.  I haven't found a way to copy DLLs using Windows Explorer, but command-line never fails.

copy "C:\WINDOWS\assembly\GAC_MSIL\System.Web.Extensions\1.0.61025.0__31bf3856ad364e35" C:\dev\MyApp\Bin\
copy "C:\WINDOWS\assembly\GAC_MSIL\System.Web.Extensions.Design\1.0.61025.0__31bf3856ad364e35" C:\dev\MyApp\Bin\