Update: The issue appears to be a common problem with the vacuum actuated 4wd front hubs. Likely culprit is an $8 check valve. Should have one in hand Thu.
Vacuum is used to pull the hubs into 2wd - system with no vacuum defaults to 4wd. Leaky vacuum signal lets the hubs get into an in between state. Happens when engine is under load and vacuum signal drops. Switch to 4A mode and the issue goes away.
