1993 Chevy Suburban Transmission

A little while ago I was driving home from out of town in my '93 Chevy Suburban (K1500 4wd). As I got off the expressway and approached home (fortunately) I found I was unable to accelerate past about 30mph (it won't change gears at that speed and will just rev with further pressing on the gas pedal). That was about 3 years ago. I've tried driving the truck around since then and the engine runs just fine, but it still won't get up past 30mph (not surprisingly).

The local garage we usually use wanted a few thousand dollars to replace the tranny, but that's not feasible right now (in medical school with no job).

I'm trying to figure out if there is something else I can do to diagnose and fix this transmission. I realize that rebuilding a transmission is hard, but I can do that if needed. I may just need to tear it apart and see what's going on inside, but I'm hoping there is a simpler cause.

Once I'm out of medical school and making money, I plan to rebuild the whole truck into a medical response vehicle for rural rescue, but for now I just need to be able to drive it and not have to pay to have it towed every time we have to move (which is getting costly).  

Any thoughts would be greatly appreciated!