If it's a shift transfer case, and he just switched to manual locking hubs then while the T-Case shifter is in 2HI or N then the front driveshaft will turn freely. If it's in 4HI or 4LO and you can turn it by hand with the hubs locked then I would look at the hubs not engaging.

When my T-Case is in 2HI and the front hubs unlocked I can spin that front driveshaft until my arms fall off!

John

P.S I didn't think the 1990 Toyotas came with push button 4WD! Got a picture?

This is my first time here, be gentle......
.....But play DIRTY!
1985 Toyota 4Runner
http://www.cardomain.com/ride/2066809 Check this if you want to see her!