5J/5U interchange

Are most parts interchangeable between a D2 5J9819 and a 5U? Particularly the tracks and undercarriage

Can you supply the 5U serial #? Parts interchange is somewhat affected by that. For example at around 5U12723, the two-piece clutch shaft was introduced, and a side impact was that the track frames were modified for length, so the rails became 33 link instead of 30.
That written, many/most parts will interchange

It's a 5u6276

Rollers, idler spring and idler assembly (although check idler diameter because larger idlers require more links in the track), tracks, if the link count is the same, sprockets, cross-shaft.
Track frames, like the tracks, should be checked for length, but as far as I know, they'll be the same length if you have the default frames (30 link tracks).
So, in a nutshell, the variables are idler diameter, track frame length, and # of links in the rails. Everything else should interchange
My 5U13753 has standard diameter idlers, and due to its serial #, has longer track frames to accommodate the two-piece clutch shaft (which increases the distance between the clutch and the sprockets) - this makes it a 33-link rail arrangement. Other options are 30 (yours), 31, and 35 but most (if not all) are after your serial #