AutoTrans Won't Shift Into Reverse Or Park, Otherwise Operating Fine. HELP!?

What is your question?

The vehicle is a 1977 GMC MIDAS RV (not in the vehicle list for selection) with the stock autotrans. It suddenly would not go into park when I stopped to get the mail and I later discovered it would not go into reverse as well. The fluid is fine, clean, right level. 57.8K original miles....it is not worn out. It drives fine and all other gears shift smoothly and there is no sign of slippage. I am thinking maybe linkage. I am just learning to walk again, by GOD's grace, and am thinking of getting my hands dirty. This is my only transportation and will soon be my only home the way things are looking. GOD's will be done. Need some insightful advice, if no one minds too much. : ) THANKS in advance. Scott in Bloomington, IN ...genesis_ark@yahoo.com

What makes this problem better or worse?

It is constant , nothing changes it.

How long has your 1990 GMC G3500 had this problem?

Since it happened several months ago. I drive as little as necessary now.