Truck is a 87 SR5 TURBO 4runner automatic. I believe the trans is a a340h. Having issues with 4wd.while in 4wd (high and low) and climbing the rear wheels spin only. Front driveshaft kinda twitches like it wants to move but doesn't. Jack the truck up off the ground and all 4 will spin. I've verified front drive shaft, differential and manual hubs are all in good working condition. And 4wd light does come on.