Sounds like a tire size problem if you have to go into reverse to get 4wd disengaged - been there with a mid-80's subaru. Even same size but mismatched brand and wear could cause a prob.

Does it roll with little resistance when driving straight on hard surface and in 4wd?

2006 Tacoma Crew Cab 4x4 Auto TRD Off Road
2002 4runner SR5 4x4 79K miles (Sold)
1993 Pickup Ext. Cab 22RE 4x4 82K miles (Sold)
Park City, Utah