Has anyone had any problem with frozen caliper mounting bolts on the rear of a 99 TXT 200? Any ideas on how to free them up?

Penetrating oil first. Then buy a cheap 3/8 drive impact wrench. I found one at a discount tool house for $15. Hook it up to an air pressure regulator. Set the pressure to about 2 bar or 30 psig. Slowly increase the pressure till the wrench makes good impacts. Should work. Just be careful and make sure the allen wrench is seated fully. If the hex socket on the bolt is ruined you can clean it out with a good solvent and epoxy an allen wrench into it. Make sure that the allen wrench socket adaptor sits straight and use a metal or silica filled epoxy. Once the bolt is out you can heat the epoxy with a torch to get the bolt off the socket.

Good Luck.

Thanks for the tip. I've got it soaking now. I'll let you know how it turns out.