when does it slip? Under any load or at high speed like 3rd gear and up?
What does the TF fluid look like? Red & transparent, still? Does it smell burned at all? Any evidence of coolant/antifreeze contamination?
Make sure it's not running low... It should be at the 'MAX' level with engine running & trans @ operating temp, equivalent of driving around the neighborhood several mins in 2nd gear
Any auto parts store will sell a kit that includes filter, seal and gasket. Brand name is irrelevant but OEM DELCO parts would be a good choice. They sometimes ask deep pan or shallow pan, you want deep pan filter or kit. Refill with 6 quart delco DEX-6 for the simple sump remove & replace, another 6-more quarts if you are going to flush out the old TF with the engine idling.
I would also refrain from using additives until you have the correct TF @ the correct level with some mileage. Open the old filter and take pics of anything suspicious: anything from silver flakes to bronze slivers and clutch material. Pics & documentation of everything on the bottom of the sump & attached to the magnets.
Since 4x4 check fill level on the transfer case and proper blue AutoTrac fluid also. Just pull the fill plug and if it's not coming out blue, stick something in there to verify color & how low it is. look on top of and around the tailshaft/rear housing for any cracks or leakage where it may have the pump rub problem.
According to the w/s manual there is a 2nd & 3rd gear whine from low & reverse roller clutch inner race & rpm dependent whine can be torque converter, oil pump, or the speed sensor contacting the rotor if sound is @ rear of the unit, i.e. Extension housing. All of which can be diagnosed: TC using the brake while trans in 'D', oil pump when coorelating line pressure w noise & then shimming the speed sensor away from the rotor.