Transmission issues

I have a 2004 Volvo xc 90 with 140k miles on it.
Forward gears and everything it runs like a champ
still but intermittently in reverse it will clunk and
Come to a dead stop. It scares me it sounds like I
hit soemthing. I am a fairly car savvy guy and do all
my own work and it has me stumped. No metal
shavings or anything in transmission fluid, shifts into
reverse fine and sometimes no issues at all. But
randomly it will just lock up in reverse. It doesn't
pop out of gear because I cannot rev it when it does
it. It literally sounds like I'm against a wall and just
can't reverse anymore but I'm not. If I try to give it
gas the motor just bogs down. I shift into drive and
drive away fine as if no issues. It really has me
stumped! Thank you all