Stuck In reverse

I live in the UK and have a 1990 GMC vandura 2500 conversion van with auto box, hence garages who know anything about this vechile are not that easy to find. The van is not used that often only having covered 2000 miles in the last year. The problem I have is that the gear selector will move out of park into reverse but not beyond this into drive etc. Additionally ( and I don't know if its linked) the last time I took the van out for a run the speedometer was fluctuating wildly. If I select reverse and while holding the brakes then rev the engine the speedo will fluctate upto as high as the 60mph mark. The van has been standing for three months on an incline with parking brake on and the selctor in the park position. I understand that there is a lock on the park position of the selector but could this also effect the selctor from passing through the reverse position and into drive. I couldn't find anything in my Haynes manual relating to this. Any help is much appreciated as I'm stuck in my driveway with no where to go!
Thanks in advance.
Neil Butler.

Stupid question I know but sometimes the obvious escapes us. Is the transmission fluid level correct? The only time to read it is when the transmission is warm. The fluid should show non the less, even when cold. The parking pall will not move but the linkage can be out of wack. Reving the engine up while in gear with a foot on the brake does more harm to the transmission. so quit it!

Thanks for the reply,
Yes fluid levels all appear to be OK. My haynes manual covers the linkages at the transmission end in quite good detail but does not show details of whats at the column end. I will check and rebuild the linkages at the transmission end but any info on if there are any locks etc at the column end that could be causing this problem would be appreciated.
Thanks again.
Neil Butler

I had an older GMC Van and discovered (Through a transmission shop) that a nut inside the transmission became loose. This prevented the shift linkage from working properly. The column shift would rotate like a clock dial and engaging any selection was hit and miss. Luckily the nut didn't come off and ruin the transmission. In that case the oil had to be drained and the pan removed inorder to see inside. This would be the opportunity to change the filter inside as well.
* I can only guess at what the problem could be. Changing the oil could be futile if work is required later but changing the oil and replacing (or cleaning) the filter may resolve the issue.

Solved my problem and now I feel really stupid :loser:

The wooden face plate that had been added to the dash had twisted and warped over the winter months and was causing the shift indicator bezel to bind. I was always taught to never force anything so didn't even think to look at this. It wasn't until I started to remove the dash that I realised that the shifter had become free.
Now I'm a happy chappie again :smile:
but thank you for the suggestions anyway.