solenoid??

[newkid17]

ok so ever since i replaced my front diff. i have had to let off the gas at 4K for it to shift into second. and when im running down the highway at a steady speed it will kinda jump and bulk around. i was talking to my tech teacher about it and he said that it is probably a tranny soienoid( duty silenoid i think he called it) i also noticed today that if i start out with the shifter in 3rd it will shift into 2ed fine.??? any advice will help..
thanks, Bobby