The Tran on my van (95 Chevy G10) started slipping when shifting to 3rd so I replaced it with another I pulled out of a 96 Astro van. This one does the exact same thing. Do I have a computer problem? How does the speed sensor affect shifting? Replaced fluid and filter. Did I swap one bad tranny for another?
Since the 3-4 clutches in the 4L60E are very weak, it is entirely possible that you got another bad one. Since you have already made one swap, I suggest having a trans shop check the line pressure/line rise on it. A scanner and pressure guage should tell you exactly what you need to know.