Van shakes when I reach my speed and need to stay at that speed.

The problem can be caused from a faulty injector, wiring harness, or power train control module. The only way to test the system is to have the van running rough when taken to a repair shop.

Hi there ,had same sort of problem with my b250 v8 5.2 1991 .changed the distributor cap leads ect ,no joy checked the torsion damper (lower crankshaft pulley) all was well had a new ignition switch to fit no change looked at engine mountings and the drivers side was shot to hell,changed this for new and the problem went away,a few months later the check engine light came on and your problem came back then i found the throttle idle motor on the carb was knackered changed that now the van runs the nuts,hope that i may have been a help all items can be purchased from auto parts whare house or rock autos for reasonable money.regards ps check compression pressures ,last resort or first port of call before you get stuck in too deep.