Your transmission is starting to slip sounds like. Maybe Torque Converter as well. Has it ever been serviced would be the question. I would say deal with it. When having to switch just go to park first from drive before going into reverse. Letting out a little fluid and adding some Lucas Transmission Stabilizer may help. If it gets worse here are your options. 

1. Have a mechanic you can trust do a partial service on your transmission. Don’t flush it or do a full fluid change. Have them pull and clean the pan. Put in a new filter, a bottle of Lucas stabilzer and if possible strain and readd what fluid was in the pan. If thats not possible top it off with new fluid. (99% of the time a full flush service will kill an older transmission) 

2. Find a used one from junkyard and have it installed. Issue here you are looking at something 25 years old. Sometimes mileage is unknown, how long it’s been sitting etc... and finding someone to swap it who knows what they’re doing and won’t overcharge you. And while most junkyards “guarantee” used engines and transmissions, sure if it goes out in a month they’ll gladly give you another used one. But you gotta pay get it swapped again. 

3. Get your transmission rebuilt. Most times this isn’t worth it, can cost way more than the cars worth. For a long time I was lucky, I had an older gentleman who had a transmission business in my neighborhood. He did one for me in 92 cost 995, one in 2005 cost 1295 and another in 2012 cost me 1595. He did good work and stood behind it. Since he passed away cheapest rebuild I can find is almost 4,000. 

Make sure the transmission fluid is topped up, and check the condition of it. It should be clear, bright red, and not smell burnt.