99 durangoTransmission shifts hard

I have a 1999 Dodge Durango. i just bought it a couple of days ago. The first night i got it, the service engine light came on. I had it checked out and they said it was the 2nd oxygen sensor. latter that night It started, it started making a load clunk noise when i shifted the trans from drive to park or reverse to drive etc. My wife drove it today and it didn't jerk and make that loud noise when she doesn't use the parking brake. could the parking brake have something to do with the jerking from trans shifting will the truck is not moving. And I took it to the shop and they said that the rear end had SLOP in it. when they turned the drive shaft, the wheels didn't spin in opposite directions exactly at the same time but it was close. and you could hear the gears or something clicking in the gear shifter or whatever that is between the trans and drive shaft. i traded in a car that was worth around $2000 and gave $3000 cash for a 1999 durango with 104,000 miles. should i sell for more or keep it ????????

It's A Day Latter And I Find Out The The Transmission Slips And Then Jerks When Going Up A Very Very Steep Hill After Just Being Started In The Morning. After It Warms Up It Doesn't Jerk Or Slip Going Up A Steep Hill??? Help Help Help???