Does the truck make any kind of clunking or popping noise while driving? Like if you accelerate and then let off the throttle is there a clunk? If so then I'd start by inspecting the U-joints. If not then it's possible there's a problem with the solenoids or varnish from lack of service before you bought the truck causing them to stick/operate slowly. I don't believe replacing the solenoids is a particularly hard or expensive job, but if you can't pinpoint the problem yourself then I'd look for a reputable trans repair shop in your area that can do some additional testing/diags to figure out the issue. Could also have your local dealer diag it but they may charge more.