L3500 issues help needed

[Mechanic90]

I inherited an l3500 a few years ago used it around my fathers acreage until he sold it . Around the time he sold the land the tractor decided it wanted to be stuck in creeper gear . I can move the high low shifters both ways with no change . What can I do? And , what oil filter can I use? Is there a non zennoh equivalent my assumption was the l275 ? L