Re: How do I fix my broken trailer hitch???
Mobile welders are about twice the cost of a normal you bring the work to the welder,so your looking at $200 per hour plus so lets just throw that idea right out the window.
In my not so expert opinion with the amount of weight you have I would purchase a coupler similar to what ezmobee posted bulldog is an excellent product so if you decide to go with them you will have no worries.
Use your angle grinder and remove the weld, try not to remove much of the base metal but there is only so much you can do, but remember its ok to remove steel from the old coupler instead of the tongue since the old coupler is being discarded anyway so lean on the old coupler heavily instead of the tongue, don't lose hope once you see the metal through the grind flake off, STOP(trust me when you see it this will make complete sense) and move to a new spot to grind once you have . Your going to need a decent sledge hammer to start beating the old coupler off the tongue and you may want to think about using a short piece of angle iron or flat stock to lean on the old coupler, I don't like using a chisel or anything sharp in this specific application since it can go into the base metal(tongue). Once the old coupler is removed you need to change discs on your grinder to a flap disc(very common and can be purchased at Lowes or Home Depot) and us this to clean the tongue up once the old coupler is removed when you have pretty shiney steel then put some rustoleum primer on it and slide the new coupler in place. Now I would want the new coupler to be welded about an inch behind (towards the boat) the old weld so if that means you need to cut a bit off the tongue so be it and if you can go ahead and cut some off but the new coupler may have a longer sleeve then the old one so we will have to see. Use a marker to ring around the tongue where the sleeve ends, and use the flap disc to remove all paint and debris a half inch on either side of the mark(all around the tongue), also use your flap disc to clean paint from the end of the coupler and put the coupler back onto the trailer.
Now what I would do as a temporary solution to your situation is once you have your new coupler ready to be welded is drill a 3/8" hole midway between the ball lock and the end of the sleeve, install a 3/8" grade 5 bolt and a nut with a lock washer and tighten one turn past the complete collapse of the lock washer only this is important because you do not want to crush the tongue and overtightening this bolt will do that pretty effectively. take the rig to your welder and have him go to town, I think your looking at about $100 total depending on minimums for the setup with all prep work done by you, once he is done take it home and repaint it.