Van was ran on low oil, and is now knocking even though the oil is full.

Sadly, if it truly was three quarts low, and you really are hearing the engine internals knocking, there is very little that can be done cheaply and reliably to save your engine. It is possible that you "spun the rod bearings" on the bottom end of the motor, and if so, changing them wouldn't be as much as a new (used) motor, but its a real crap shoot to determine just how much damage may or may not have been done to other parts of the motor. Even saying all that, I just noticed that the noise stops after second gear. If the sound stops because the speed of the motor is lower in a higher gear, we could still be looking at internal engine damage, but that's why you need a good mechanic you can trust. Get your van to a mechanic who you trust very much, and they may be able to steer you in the right direction. Hopefully the knocking you hear isn't related to the engine internals, but there's no way to diagnose properly over the internet :( At the very least, even if the mechanic thinks the damage is done, you can drive it till it stops completely (usually relatively safe to run them till they "blow" if you don't mind being randomly stranded) Sorry to hear about your problem, Good Luck :)

Your least expensive fix. Put in a bottle of "Prolong Engine additive". You should notice engine noise to be about 10-20% quieter (in your case) and the coating will stay on the mating surfaces of all the running fits overnight for an immediate lubricated interaction upon start up. I have used it and the transmission additive for about 25 years (until the sealed trans on our 2008 mini) getting close to 200K miles without rebuilding transmissions like I was doing before. Change engine oil every 20K (unless it nullifies your warranty) putting in the additive to the full mark. REF: Our vehicles bought used with less than 20K miles on each: 1990 Eagle Premier, 1992 Dodge Monaco (wife's car), 1996 Concord, plus four mini vans. At retirement, bought new T&C Mini and use it there (140K miles). Also use it in 1966 Corvette, which I bought new, that sat for 31 years. Just bought 1999 300M 24K miles.