MOT man says I need to do my swivel seals: one looks like it leaks, but the other's OK, but there's no oil in the diff and teh bearing seals are shot. Both swivels are shiny with no pitting. Do I need to replace the swivels, or will I get away with just the seals? It's a 1991 Def 90 and I think the swivels are original.