1987 GMC Van Do I need to rebuild my trany?

Hello, New comer here.

I just bought a van for $600 (cheap I now, Had to be something wrong with it, right?). First thing I notice was that going from first gear to second, the van would jerked foreword in a roughly fashion (1st gear revs vary high before shifting). I can nearly eliminate the thump if I release the gas right before it shifts. As I release the gas, it'll shift when the RPM's are declining. If I timed it wrong and it shifts when the RPM's are high, I'll get a thump.

As it shifts from 2nd gear to 3rd, it would bounce forward and back as if it was a stick shift ie. If you would release the clutch to quickly. Shifting from 3rd to 4th is flawless. I get the same thump from 2nd gear to 1st, but not as violent.

Do I need to get the trany rebuilt?

How much driving time you think I can get?

Thanks