The not going into gear problem is probably related to being low on fluid. If it is leaking that much, it will shift funny.
Make sure the trans cooler lines are tight on the side of the trans. Also, check the seal on the dipstick tube to make sure it is not leaking from there. GM has an updated tube with a boot type seal and a locking dipstick tube to prevent leaks. There was a recall about transmissions building up heat and pushing fluid out of the dipstick tube. It would then land on the converter and burn the truck to the ground.
you might check to see if the seal inside teh tailshaft is leaking. There is a weephole in the driveshaft of some trucks that will leak if the seal is going bad. I believe there is an article on
www.73-87.com .
Other than that, the only other places that I could think of that may be leaking are the front and rear trans seals.