1999 Dodge Durango Transmission troubles (Help diag)

Just bought this 99 Durango 5.2L 4X4 with a bad transmission. The guy I bought it off of said in November (I know we had alot of snow around here last November because I was out riding my snowmobile) they was driving it and heard a loud bang, they then drove it home after the bang and after they parked it the transmission would not work anymore. That's all the info I could get out of the guy because he was not really a mechanically inclined person.

I know nothing when it comes to transmissions myself. I can pretty much do anything with an engine but just never had any rebuild/diagnosing experience when it comes to transmissions.

What it is doing
Park
works no need to use blocks

Reverse
you can hear a small clunk like you would if it had a bad u joint.

Drive
only thing is you hear a small sound (very quit) like maybe a pump running or something. All I can tell you on this is you can hear something is going on. Could be just the torque converter though

2nd 1st
same as drive

I plan on pulling the dust cover and checking out the converter to make sure they didn't just shred the bolts. This would be my limit to diagnosing a transmission issue lol

Any ideal what it may be or anything that you could suggest for me to check would be greatly appreciated. If its simple enough (something like replacing converter ext.) I probably just do that myself but if it needs something more then I will just take it to a tranny shop. I just don't want to get ripped off and charged for a rebuild if it doesn't need it. I would like to have some ideal what it is because as of right now I have no ideal and a shop will just rip me off more then likely.

Thanks
Jo

Edit:
I just listened to where the clunk was coming from, had somebody put it into reverse while I was under it. It is coming from the converter area, so is the quit noise I was talking about. When I get time I will be pulling dust cover.

I also noted the tranny fluid level on the dipstick (fluid looks almost brand new by the way) then started the truck, placed in neutral then checked dipstick again and it was lower. So, the pump seems to be working.