I have a 1996 Blazer that the check engine light has came on in 4 times in the last month. The main code is transmission control system malfunction. I have only had this Blazer for 2 or 3 months. Fluid was low so I added and thought that would solve the problem but it hasn't. It shifts hard through first and second gears (rpms will jump up to 2,500 and then drop back down when it does shift). After 40mph it shifts pretty good and the rpms hold steady for the most part. I was told to try to replace the filter and fluid and put in a new solenoid. Does that sound right? Any tips on how to replace a solenoid?

Thanks!