TH400 no reverse
 

I have an 87-88 GMC 3500 crew cab with a 6.2 and TH400 that came with my house as a plow truck. A friend borrowed it to plow his yard last winter and managed to break it. All the forward gears work fine but it doesn't do a thing in reverse.

Someone told me there is some server in the transmission that sometimes goes out and is a fairly cheap and easy to change part and would cause this. The concept sounds very nice compared to taking it into a transmission shop, but I don't know anything else about this or even if it's a good theory.

Anyone know anything about this? Everything is breaking at the same time, so it'd be really nice if I could fix this one without having to take it in for a transmission rebuild.