Thanks!

I Googled the hell outta those numbers and looks like there are 3 sets of frame splash shields. 82-89, 90-91 and 92-93.

DRIVERS SIDE (LEFT)
4215565 fits 82-89, 55022667 fits 90-91, 55254625 fits 92-93

PASS SIDE (RIGHT)
4215536 fits 82-89, 55022666 fits 90-91, 55254624 fits 92-93

I found I had an old 77 Dodge Truck Service Manual on CD. Doesn't show anything as far as these splash shields are concerned. It DID show that diesel trucks had a ton of extra insulation inside the fenders, on the engine side of the firewall and behind the kickpanels.