55 transit van?

Do you mean it won't go in to gear or jumping out of gear? Not sure on yours but there was a pinch bolt on top of the gear selector mechanism which used to come loose,common problem,also where the selector shaft on top of the g/box passes down through the housing,I would give it a lubricate,the bearings under the small oil seal are bad for drying out and the shaft seizes up,both items are worth a look,good luck.

cdc
March 2014