I think its time for a rebuild!.. You have to have alot of water enter that transmission to cause a problem.. Once a clutch slips... Its all over!! I had a 700R4 in my blazer , after driving it, the tranny let go and only had reverse.. this is because of the overdrive unit inside the transmission.. The overdrive clutches can not take a great deal of abuse; nor can they take large tires! If you've ever seen an input shaft on that TH700R4, its very small and all that engines power goes through that itty bitty shaft.. My opinion. Your trans is junk..get a TH350 or TH400 and appropriate adapters and swap them out and call it a day....
overall: your overdrive clutch pack has slipped beyond repair (is the fluid dirty or watery?)--i suggest not rebuilding it if you use big tires or tow alot...its not a strong tranny..