Sounds like the clutch pack might have failed what oil is in the transmission if it's got red dexron II atf your screwed the transmission will need stripping and the pack plates and seals replacing because the plates will break up. This transmission needs Tractran JD40 oil ive repaired plenty of hyster transmissions where a so called technician has used atf in them

It has been my experience that when this happens, you most likely had the snap ring break on the end of the Clutch pack. If you pull the dipstick out and look into the transmission while in reverse, you will probably see or smell smoke.
It sounds like the clutch plates if forward are not releasing. So the transmission is actually try to go Forward and revere at the same time.
Seen this many times on trucks that got stuck in a hole or something and the operator tries to "Rock" it back and forth to get out. Will break the large Snap Ring on the End of the clutch pack.