If your LED and LCD work fine on the 4553, then that PIC is working fine.

If your 4550 can talk to another 4550, then that PIC is working fine.

Are you using the right TX/RX pins on the 4553?

There's really not a lot of possibilities here. The 4553 is EUSART compatible with a 4550. It is basically a 4550 with a different name.

Do you use different wires?

Do you do a simple drop-in substitution for the 4553, or do you have a separate breadboard circuit set up?

What about a heartbeat LED?
Main:
LEDout = 0
PAUSE 500
LEDout = 1
PAUSE 500
GOTO Main
END
If you're using a separate circuit for the 4553, does this do a 1 second pulse?
(I was thinking of a timing issue, LED and LCD might work fine, but EUSART would be more finicky)

Just trying to eliminate all possibilities I can come up with.

Robert