Duckburg

Hello Everyone, I'm new to these forums. Sorry my first post is a technical question, but I wanted to get some input. I was given a 1996 chevy silverado extended cab 2wd pickup as a gift. After driving it for a few days, i'm starting to notice the transmission is acting weird. From stop/first to second, the transmission will kick a little bit. Not terribly hard, but it's noticable. When shifting from 2nd to 3rd, there is a delay. It doesnt feel like the transmission is slipping really, but i supose it could be. The RPM revs about 500 or so RPM, then the transmission will catch and the rpm will drop. It's subtle, but noticable. There are a BUNCH of aftermarket parts on this thing. K&N cold air intake system. Some sort of exhaust. After market carb etc. I'm really not sure if it's "suposed" to be acting that way or what. When the delay in shift happens, there is a hesitation in acceleration. I just wondered what steps i should take before trying to put a new transmission into it. Has anyone expreienced anything like this?