If your vehicle is showing red or pink transfer case fluid leaking from the middle of the vehicle, here are the most likely causes and the parts you'll need to fix each one.
The seal where the driveshaft exits the transfer case is a frequent wear item. Over time, the rubber hardens and allows fluid to seep past the driveshaft yoke, slinging pink/red fluid under the vehicle.
If the transfer case fluid was recently serviced, the drain or fill plug may not have been tightened to specification, or the crush washer/gasket was reused instead of replaced.
The seal between the transmission and the transfer case can fail, causing fluid to leak from the mating surface or weep hole between the two units.
The sealant joining the two halves of the transfer case housing can degrade over time or fail due to improper application during a previous rebuild, causing a slow leak along the middle seam.
Internal components like the fluid pump can wear a pinhole through the housing over time (a known issue on some trucks called 'pump rub'), or the case can crack from severe off-road impacts or driveshaft failure.