2009 Kubota RTV 900, 2009 Kubota B26 TLB & 2010 model LS P7010
4 WD actually takes more power than 2 WD so if it is not working in 2 WD, then the transmission surely wouldn't work in 4 WD. You may have a stripped spline on the front drive shaft on one or both ends. That is only thing I can think of that would make 2 WD not work but 4 WD work.