Hello, I was recently gifted this old junker truck that I want to use for my business. It needs a good deal of work but it's mostly stuff I can do myself, with a bit of research and some borrowed tools. What I am really concerned about is the transmission.

It doesn't downshift into 1st or 2nd gear if it's moving. It will shift up just fine but if I am slowing down to turn a corner or in stop&go traffic, it won't go back down to 2nd and I have to come to a complete stop to get the Truck moving again. I can move the shifter down into the spot but it doesn't click. Sometimes, and only if I slow down to under 10mph and foot the clutch just right, it'll click into 2nd but it bogs the engine down really hard and the Truck shakes. Can't be good for the engine...

Any ideas of what could be causing this, and what I'm gonna have to do to fix it? I'm hoping that something as simple as a fluid change could fix this but I doubt I'm gonna be that lucky. I cannot afford to go to a shop, I've been doing my own maintenance on my cars lately to save money. Brakes and oil ect. Are pretty easy but I know neither jack nor shit about transmissions...

Edit: 1995 gmc sonoma sls 2.2 liter 5 spd manual