It's beginning to slip out of second gear (when gas is pressed down hard) and overdrive! Has anyone else had similar problems? Also - if the transmission needs to be replaced, would it be smarter to use an aftermarket or heavier-duty trans instead of the one that is made for the truck? Please help!

 

I'm guessing the tranny has been serviced regularly and the fliud has been checked and the filter repalced, being a 94 I'd guess you have at least 170k miles on it, it could just need a band adjustment but more then likely the clutches are getting worn out and it may need to be rebuilt. A transmission shop could give you a quick assesment.
having it rebuilt is usally cheaper then replacment but sometimes if you can get a good deal or a factory refurb a shop with can change them out for you for a few hundred bucks.