Q: Vehicle lost gears, can shift only in 4 low

My truck has an automatic transmission that would slip out of gear when I was coasting or driving downhill. However, it would shift back into gear as I apply the gas pedal. This problem was occurring for a while and it wasn't such a big deal until the shifting back to gear became a bit rough. I took the truck to a shop and they didn't find anything out of the ordinary except the transmission fluid that was filled a bit too much so they drained it. They also put in some TransX against slipping and told me to shift manually. The following time the problem occurred was during backing up while the truck was in the two wheel drive mode. I tried to go to reverse, but I suddenly had no reverse so I put it in 4 low and I eventually got it back. I took it to the shop again and this time they had the transmission filter replaced. I replaced fluids as well. Everything was fine until a couple of days later when I wasn't able to downshift and soon lost all gears. The only way I can shift is if I'm in 4 low. Do you have any idea what might be causing this?