So my 98 ram 1500 has been acting wierd lately. We've had a lot of heavy rain and im kind of thinking something got wet and is causing all these malfunctions.

I am throwing the following codes:
PO203: Injector circuit malfunction - cylinder number 3
PO300: Random/multiple cylinder misfire deteced
PO301: Cylinder number 1 misfire detected
PO303: Cylinder number 3 misfire detected
and PO720: Output Speed sensor malfunction

720 explains why my speedometer has been jumpin all over the map. As i came to a stop it jumped up to 120mph and its been bouncing around a lot the last day or so.

Any ideas what specifically I should be looking at? is there one route cause to all of these malfunctions?

 

I think your speed sensor is combined with the ABS sensor on the rear diff. These are fairly common failures. Just replace it.

The injector circuit, and misfire on cylinder 3 are probably related. Check and make sure the connector is seated well, and the little retaining clip is still there.

Might wanna consider changing plugs/wires/cap/rotor if it hasn't been done for a while.