Brake light and check engine light randomly turns on

This vehicle has a lot of miles but overall is a good truck. it has all three u-joints bad right now so it shakes between forty five to mph and that is it. anyway if i drive it for say more than twenty minutes on the highway the check engine light comes on and or the brake light on the dash. now i know how to check OBD1 and every time i do it flashes a 1...2 so 12 which is system normal. and again the brake light comes on mid drive. i checked the fluid level and it is fine, the e-brake never gets used and it is fully released, and there is no leaks anywhere and it brakes fine and i do not know what to do about both problems. Both of the fuel injectors are old but I have been running Lucas oil fuel system cleaner and it is been better, does not miss at all. so I do not know what the problem is, or if there even is a problem. but i am tired of seeing a Christmas tree on my dash while i drive.