You CAN try it, but it will be a disaster. As stated, the rear driveshaft also acts with the rear seal.
Can't remember if it was here or one of the other Subaru boards, someone tried it and didn't make it out of their driveway or garage.
Subaru never built it's AWD transmissions ( manual or Auto's ) to be driven wihtout that driveshaft. The FWD transmissions of the 70's 80's and early 90's were different.
We HAVE drained transmissions, both manuals and automatics, removed the driveshaft just to TOW them when a flatbed was not available, ( front up, rear on the road ).
98S, 2014 Impreza 5 dr Ltd (9 Subaru's since 1978, 7 were 5 sp) ILuvSubaru's