I have a 2000 4-Runner Limited with a manual transmission. Recently it has started slipping when in 4-high and first gear. After first gear it is totally fine and in 4-low or 2-high there are no problems.
If it’s slipping it’s usually the clutch.
Agree with Mike… there is really nothing else TO slip in a manual trans driveline but the clutch.
Stop running it in 4 hi. For a 2000 could be a major expense and limp along, unless you are willing to put at least $1200 into it.
Typically, if the clutch plate is worn, the slippage will show up first in the highest gear, not the lowest. Have the hydraulic system checked out. It may have a slow release.