We had this issue as well on the 96 RMW that we just picked up. Drove it back from NY to MN during the odd, late season heat wave. I checked for the elbow and in our case, it was in place. This may not be the fix, please let us know if it is.
After some searching, on GM trucks, there was an issue where within a certain range of speed that the air pressure would build up and not allow the AC unit to vent. I believe that most people attached a hose either direct from the output or onto the elbow and ran it down to the frame or a mount and had it downward and toward the back of the vehicle to prevent the pressure.
There are also some threads where folks are having issues with a seal on the condenser box under the dash. When the fluid builds up it is leaking through the bad seal.
There additional are some issues with stuff getting into the condenser box (leaves etc.) that end up plugging the outlet. The fluid then builds up and leaks into the car.
On our drive, we noticed the wet floor on day 2, but on the 1st day we noticed when we stopped that A LOT of water would be draining out under the car, so I believe it is draining OK. I am wondering if we had a pressure issue at certain speeds. I am going to take the box apart and inspect the inside, especially since I took a coat hanger and pushed it in there to see if there is a blockage. After doing that, I thought it may be a bad idea so a visual inspection is in store.
Let us know what you find.