Transmission Slip?

[fitzthecat]

new truck. 95 gmc excab long box 4x4. few problems, trans slips in reverse, only when moderate throttle applied otherwise reverse works fine. saw a post before i became a member about a part# for a valve and seat? but now i cant find post. believe i have a posi rear when driving in slippery conditions and 1 wheel is spinning a severe clunking is felt is this the posi working or is this something to worry about. have a coolant leak and thanks to your forum have decided it is time for a new intake gasket. i guess i will need valve cover gasket thermostat gasket intake gasket and rtv silicone for repair. 95 gmc 5.7 auto (4160e?) 221000 kms.would also like to do new trans fluid and filter but would like to find out more about slipping reverse before i do this. o2 sensors, anybody recommend best place and cheapest.