Take it apart and have it welded. OR, an emergency fix I did one time that
held for months, before I finally remembered to weld it back together:
Get one of these bits:
http://www.amazon.com/gp/product/B000CO9NUC/ref=ox_sc_act_title_1?ie=UTF8&psc=1&smid=ATVPDKIKX0DER
carefully tap the blunt end into the stub in the floor halfway, using
vice-grips to hold the stub upward. Reposition the vice grips so that it
squeezes on opposite corners of the now-inserted bit, to prevent it from
going further. Remove the shifter knob from the broken handle, place over
the serrated end of the bit, and drive it down over tightly using a hammer
and block of plastic or other non-metal hard object. You must seat both
parts together tightly. Install the boot and knob. The size of this bit is a
nice tight fit for the inner diameter of the shifter and joined correctly
will hold very well until you get to a welder.