Are you using the TX pin for XOUT? If so, yes It may give you some weird problem, because XOUT may try to modify the TRIS setting, and/could disable the USART.
Try
RCSTA=0 before XOUT
RCSTA=$90 after Xout
and see how good, worst it is.

From what I know, there's no good reason why MODEDEFS should solve the problem. If it wasn't there, you would probably have some compilation error but not much.