Could be a pump problem or broken planetary. Regardless, the transmission will need to be disassembled.
I had this problem in my 2007. the forward sprag was broken. i still had 1st, 2nd 3rd and reverse but no drive.
If Reverse works and Drive doesn't in a Safari Van, the problem is in the transmission. The belts for the forward gears are either stretched too much or broken.
When a Toyota 4Runner will not move in reverse, it could be that the reverse band in the transmission is broken. Another possibility is a problem with a range selector. Both problems would need the services of a transmission expert because the transmission would have to be pulled.
The transmission is broken. Take it to a shop.
do you have all forward gears? if yes, you have broken reverse gear (one turning the spin into reverse) in the transmission, if you miss also some forward gears it shifts rough, tranny is to be rebuilt... either way it's expensive fix
Low fluid, broken transmission.
One possibility is a brake caliper that has a broken or missing bolt. Or something has broken inside the front axle.
The input sunshell splines have stripped or the hub has broken off. This will give you 1st + 3rd gears only with no reverse, 2nd, or 4th gear.
back brake springs weak or broken! or a brake part is broken.
You may have a broken spring which disengages the transmission allowing it to shift or it is bad linkage going to the transmission.
It truly depends on the trans - auto or manual? Chev or Dodge? However, if you can go forward but reverse is gone, you have broken parts in the transmission. Really common on some GM's. Dan the tranny man