I have a 96 300 xplorer that every time I ride thru a mud hole and get it wet the tranny starts slipping.If it sits for a couple minutes it's fine again. Does anyone know what could be wrong?  

older polarises will do this. the belt gets streched and starts to slip. however if its only when you go through mud, it could be the seal/gasket. so check that first. however, i have a 97 300 xplorer that would just plain power out in the mud, it felt like a loose belt, but it wasn't. i ran some fuel system cleanser and octane boost through it and replaced the fuel filter and that fixed it, id try that first if you arent 100% sure its the tranny, cuz parts are epensive. the filter and fuel treatments cost me a total of about seven bucks. email if u have questions to kjohnson_acdc@mustang.com  

Ok, belts are actually getting wet and slipping.  When they dry out, they stop slipping. Pretty cut and dried. No stretching that goes away, no mechanic-in-a-can.

My $ 0.02 worth  

yeah check the pvt transmission belt cover.
it has a rubber seal, but if you've damaged it then it will allow water in there.

and of course any belt will slip if it gets wet.