Sounds like the clutch may be worn out. Coupled with your other post regarding low speeds and bad performance up hills as well as trouble with gear changes, I would suggest that you investigate this cause first, as a slipping clutch will be inefficient and eventually you'll end up going nowhere as the van won't move. Other than this, the clutch plates may be contaminated with oil etc, which usually means replacement as well. If the clutch is not at fault, a worn selector fork in the gearbox can causes problems with gear selection. If you need a clutch replacement, I would suggest Mr Clutch if you have one near to you, they were cheapest for my Sierra and did a good job. It was cheaper than me buying the bits and losing a days wage to do it myself (and a lot less swearing).