93 grand caravan le trans lacks high gear

when I get on the freeway the van rpm's go to 5000 at 60 mph. Fluid added it works. 2 days later check gauge light on and it won't dshift into high gear. Problem last year  was relay that affected rear back up lights at firewall. How to service the trans? It's a 93 3.3 engine with 190000 miles and has been pampered.