99 chevy blazer, this is the 4th 4wd drive actuator i have had to replace. transmission fluid gets in (about teaspoon) & rips the rubber. tranny fluid levels fine. what's causing this ?

 

It is a combination of a bad vacuum switch on the transfer case and a bad input seal on the transfer case. Check the fluid level in the transfer case by removing the upper plug. If fluid pours out, you need to remove the transfer case and replace the input seal. While it is out, replace the vacuum switch.

You then need to clean out the vacuum system, hoping that the transmission fluid has not gotten all the way to the HVAC controls.