While driving my 03 Toy 4x4 , pops out of H2 into H4 all by itself,

Hello Toy onwers,
Have had Toyota's for a long time, first time on Tacoma world, always get work done at dealerships, until now. I have a 03 TOY Tacoma 4x4 access cab 5spd manual, at 50-75 mphs it pops out of H2 into H4 without touching the shifter! Dealer says nothing wrong, happens to often to be nothing wrong. it only has 40k miles on it . I bought it off showroom floor in 03, with 30 miles on it, any body else every have same issue, thanks frank