800 miles pulling 3 horse trailer/8 foot living qtrs. (9000lb. ) and after getting off sustained interstate driving to get fuel felt a rapid shudder through 1st and 2nd gear. Went away. Noticed it once more when exiting to stop again. Start back off it was gone. Next day after same deal its worse. Even after fueling and starting back up truck shudders rapidly through first and and second. Like posi shudder but I don't think so. It does it dead straight also. U/Joint?