not wanting to be the bringer of doom, but it sounds like either the clutchpackor brake band between second and third gear has burned out, causing a 'flare' between gears.. (most likely the clutchpack)
you might be lucky, and it could be something else, but its unlikely...
very odd at that kind of milage though........... the 4L60E is normaly good for at least 120,000 miles, and often can go on to double that, even my 700R4 (basicly the same box without the electronic controls) is perfect at 138,000 - and they are known for giving up earlier then the 4L60E
the classic first symptom of low fluid is a reluctance to go into either drive or reverse, or droppping in to neutral then back into drive when coming to a standstill...
try checking the fluid (as steve sugested) with the following method,
get the engine and gearbox up to operating temp (not gonna be fun with 2 gears)
starting in park, engage every gear in turn (including nutral and the gear holds) 3 times, then with the engine still running, in either park or neutral, check the fluid level..
also have a good sniff of the fluid - if it smells strong and pungent, its bad news.