Go to my thread on the 1st gen page, or on my new member thread. I had the same problem on a 97 I bought. I was lucky enough to be able to take it back and recover my money.
You can purchase a new frame from Toyota for $2000, or you can get a rear frame clip from a pickup wrecked in front and weld the clip onto the good part of your frame. You should be able to pick up a frame clip for $300 or so and have a welding shop weld it in for you. Of course it would entail some time in removing the bed and all attached parts and making absolutely sure the clip was welded in true and square, then replacing everything. Might be worth it if the rest of the pickup is good.
Some folks might just try patching the frame by welding in steel straps, but you can't weld to steel that's not solid. The rust just keeps on going. To do it right you have to remove the rusted metal and weld to solid clean metal.