Peter:
This EXACT symptom occurred to my 86 Transporter about a year ago. After
I tore down the trans axle and pressed off the gears from the main shaft
did I find the Synchronizer hub for 3rd/4th had broken.MAJOR pain in the
gazotska.Requires a special tool to get into the gear box,luckily I was
in good with the service manager who let me borrow it.
(onecaseofbeerfortheshopreward).
 email me for more details.
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86Doublecab pickup
Ken Lewis  kdlewis@juno.com
85GL
60 356(almost finished)
61 AH Sprite (for sale)