1985 Truck, Transmission rear end question

[Txnrbl]

New to the board, have been looking around some trying to find a starting point for my problem we will call it. Long story short. I am getting back my 1985 Chevy that I bought when I graduated high school from my parents. After I moved out I needed a more economical car commuting the distance for work after high school. My step-dad and brother have been "tinkering" with the truck some. Well, my step dad had some transmission work done by someone that they thought was good at this work. I have not gone to pick up the truck yet (will be putting it on a trailer for the travel back to my house) but the best description I can get about the issue is:

"When you accelerate the truck will only go about 55mph, its either a transmission or rear end gearing thing".

I know now much to go on, and this is not my expertise so I will probably have a local shop look at this. Before I do however I would at least like some input so I can at least have a little knowledge on what I should be looking for listening for from whomever does the work, if it goes to someone else.

Thanks for any input.