I keep breaking 208f transfer cases off. any idea's

Do the drive shafts still have the rubber screw on seal to protect the shaft splines? If it does try removing them to allow more freedom of travel when extending and compressing the driveshaft. Also make sure it has a lot of grease on it so its not sticking. We never had any luck running the 208 transfer cases in any mud truck(fast track racing) so we quit using them after breaking several and use either the 1345 or 1356 case if in need of the very low ratio but with high horsepower/torque we run the good old 205. the 205 may be heavy(but helps with weight distribution) but they always held up for me.

Did you ever find your problem why it was breaking

kinda! bump stops helped a lot on the front suspension. we cracked 1 in two years now. we run 2 trucks with 208 t case.

Lots of high power trucks running them but they're usually on the high side with all the low range parts removed