1985 Astro not shifting out of low sometimes

I have a 1985 astro van with 53,000 miles that sometimes will not shift out of low gear. Sometimes when it does shift it will pulse when it shifts. Then sometimes it will shift just fine like it normally should. This has just started happening I do not drive the van alot as you can mostly for trips since it is a conversion. I don't know if the transmission is computer controlled as I was told it does not have a modulator valve. Please help.