S-10 transmission bang

I have a 2000 s-10 4x4 pickup ext. Cab 4.3 v6. With 112,000 miles on it. I have had a problem with the check engine lite coming on and it saying the codes says check the transmission computer. But anyways what is happening is that when you drive it from a stop, I hits that first shift point and it bangs HARD. But not all the time or any other gear. I was told to replace the filter and then a solenoid and then the seals. Help. Also when I use my 4 wheel, it doesn't shift easily from 4 wheel high to 2 wheel easily, it also bangs loudly. HELP!