Tranny Problem

[00sahara]

Hey guys. I just got my truck back in March and started noticing this little "thunk" when I was at a complete stop and let off the brake to accelerate, and the same "thunk" right before coming to a stop. I assumed it was the tranny mount so I took it to a shop this passed Friday. When I picked it up 2 hours later they told me my tranny was slipping. I got in with one of the guys and he put it in drive and let off the brake, the truck didnt move, he gave it a little bit of gas and it popped real ugly and started moving. Then shifted roughly into second and wouldnt go into third. after a bit of driving it shifted to third, then downshifted back into second reving up the engine. Theyre saying the tranny needs to be rebuilt. Does this sound familiar to anybody? It seemed like it was driving fine before that. Definitely wasnt slipping and it would start moving once i put it in drive. The tranny old tranny mount was intact and they said that was never causing the "thunk" i would hear. although now when i stop it doesnt do that anymore. I drove it home real slow, it entered a limp mode where the tow/haul light was flashing. What could have caused this? Thanks