I know on each end of the driveshaft there is a flex coupling but there is also a single u joint in the middle is that replaceable or no I'm getting ready to do the flex couplings and figured if I have it out why not do it if I can

 

A service part for replacement u-joint is not offered by Lexus.

It can be replaced, but requires proficiency with these driveshafts.

Have a guy in Chicago who specializes in Toyota driveshaft center u-joint replacement